Обсуждение: On disable_cost

On Fri, Nov 1, 2019 at 7:42 PM Zhenghua Lyu <zlv@pivotal.io> wrote:
>     It is tricky to set disable_cost a huge number. Can we come up with better solution?

What happens if you use DBL_MAX?

Em sex, 1 de nov de 2019 às 03:42, Zhenghua Lyu <zlv@pivotal.io> escreveu:
>
>     My issue: I did some spikes and tests on TPCDS 1TB Bytes data. For query 104, it generates
>  nestloop join even with enable_nestloop set off. And the final plan's total cost is very huge (about 1e24). But If I
enlargethe disable_cost to 1e30, then, planner will generate hash join. 
>
>     So I guess that disable_cost is not large enough for huge amount of data.
>
>     It is tricky to set disable_cost a huge number. Can we come up with better solution?
>
Isn't it a case for a GUC disable_cost? As Thomas suggested, DBL_MAX
upper limit should be sufficient.

>     The following thoughts are from Heikki:
>>
>>     Aside from not having a large enough disable cost, there's also the fact that the high cost might affect the
restof the plan, if we have to use a plan type that's disabled. For example, if a table doesn't have any indexes, but
enable_seqscanis off, we might put the unavoidable Seq Scan on different side of a join than we we would with
enable_seqscan=on,because of the high cost estimate. 
>
>
>>
>> I think a more robust way to disable forbidden plan types would be to handle the disabling in add_path(). Instead of
havinga high disable cost on the Path itself, the comparison add_path() would always consider disabled paths as more
expensivethan others, regardless of the cost. 
>
I'm afraid it is not as cheap as using diable_cost as a node cost. Are
you proposing to add a new boolean variable in Path struct to handle
those cases in compare_path_costs_fuzzily?


--
   Euler Taveira                                   Timbira -
http://www.timbira.com.br/
   PostgreSQL: Consultoria, Desenvolvimento, Suporte 24x7 e Treinamento

Hi,

On 2019-11-01 19:58:04 +1300, Thomas Munro wrote:
> On Fri, Nov 1, 2019 at 7:42 PM Zhenghua Lyu <zlv@pivotal.io> wrote:
> >     It is tricky to set disable_cost a huge number. Can we come up with better solution?
> 
> What happens if you use DBL_MAX?

That seems like a bad idea - we add the cost multiple times. And we
still want to compare plans that potentially involve that cost, if
there's no other way to plan the query.

- Andres

On Fri, Nov 1, 2019 at 12:00 PM Andres Freund <andres@anarazel.de> wrote:
> That seems like a bad idea - we add the cost multiple times. And we
> still want to compare plans that potentially involve that cost, if
> there's no other way to plan the query.

Yeah.  I kind of wonder if we shouldn't instead (a) skip adding paths
that use methods which are disabled and then (b) if we don't end up
with any paths for that reloptinfo, try again, ignoring disabling
GUCs.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

re: coping with adding disable_cost more than once

Another option would be to have a 2-part Cost structure.  If disable_cost is
ever added to the Cost, then you set a flag recording this.  If any plans
exist that have no disable_costs added to them, then the planner chooses the
minimum cost among those, otherwise you choose the minimum cost path.



-----
Jim Finnerty, AWS, Amazon Aurora PostgreSQL
--
Sent from: https://www.postgresql-archive.org/PostgreSQL-hackers-f1928748.html

Hi,

On 2019-11-01 12:22:06 -0400, Robert Haas wrote:
> On Fri, Nov 1, 2019 at 12:00 PM Andres Freund <andres@anarazel.de> wrote:
> > That seems like a bad idea - we add the cost multiple times. And we
> > still want to compare plans that potentially involve that cost, if
> > there's no other way to plan the query.
> 
> Yeah.  I kind of wonder if we shouldn't instead (a) skip adding paths
> that use methods which are disabled and then (b) if we don't end up
> with any paths for that reloptinfo, try again, ignoring disabling
> GUCs.

Hm. That seems complicated. Is it clear that we'd always notice that we
have no plan early enough to know which paths to reconsider? I think
there's cases where that'd only happen a few levels up.

As a first step I'd be inclined to "just" adjust disable_cost up to
something like 1.0e12. Unfortunately much higher and and we're getting
into the area where the loss of precision starts to be significant
enough that I'm not sure that we're always careful enough to perform
math in the right order (e.g. 1.0e16 + 1 being 1.0e16, and 1e+20 + 1000
being 1e+20). I've seen queries with costs above 1e10 where that costing
wasn't insane.

And then, in a larger patch, go for something like Heikki's proposal
quoted by Zhenghua Lyu upthread, where we treat 'forbidden' as a
separate factor in comparisons of path costs, rather than fudging the
cost upwards. But there's some care to be taken to make sure we don't
regress performance too much due to the additional logic in
compare_path_cost et al.

I'd also be curious to see if there's some other problem with cost
calculation here - some of the quoted final costs seem high enough to be
suspicious. I'd be curious to see a plan...

Greetings,

Andres Freund

On Fri, Nov 1, 2019 at 12:43 PM Andres Freund <andres@anarazel.de> wrote:
> Hm. That seems complicated. Is it clear that we'd always notice that we
> have no plan early enough to know which paths to reconsider? I think
> there's cases where that'd only happen a few levels up.

Yeah, there could be problems of that kind. I think if a baserel has
no paths, then we know right away that we've got a problem, but for
joinrels it might be more complicated.

> As a first step I'd be inclined to "just" adjust disable_cost up to
> something like 1.0e12. Unfortunately much higher and and we're getting
> into the area where the loss of precision starts to be significant
> enough that I'm not sure that we're always careful enough to perform
> math in the right order (e.g. 1.0e16 + 1 being 1.0e16, and 1e+20 + 1000
> being 1e+20). I've seen queries with costs above 1e10 where that costing
> wasn't insane.

We've done that before and we can do it again. But we're going to need
to have something better eventually, I think, not just keep kicking
the can down the road.

Another point to consider here is that in some cases we could really
just skip generating certain paths altogether. We already do this for
hash joins: if we're planning a join and enable_hashjoin is disabled,
we just don't generate hash joins paths at all, except for full joins,
where there might be no other legal method. As this example shows,
this cannot be applied in all cases, but maybe we could do it more
widely than we do today. I'm not sure how beneficial that technique
would be, though, because it doesn't seem like it's quite enough to
solve this problem by itself.

Yet another approach would be to divide the cost into two parts, a
"cost" component and a "violations" component. If two paths are
compared, the one with fewer violations always wins; if it's a tie,
they compare on cost. A path's violation count is the total of its
children, plus one for itself if it does something that's disabled.
This would be more principled than the current approach, but maybe
it's too costly.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Fri, Nov 01, 2019 at 09:30:52AM -0700, Jim Finnerty wrote:
>re: coping with adding disable_cost more than once
>
>Another option would be to have a 2-part Cost structure.  If disable_cost is
>ever added to the Cost, then you set a flag recording this.  If any plans
>exist that have no disable_costs added to them, then the planner chooses the
>minimum cost among those, otherwise you choose the minimum cost path.
>

Yeah, I agree having is_disabled flag, and treat all paths with 'true'
as more expensive than paths with 'false' (and when both paths have the
same value then actually compare the cost) is probably the way forward.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services 

On 2019-11-01 12:56:30 -0400, Robert Haas wrote:
> On Fri, Nov 1, 2019 at 12:43 PM Andres Freund <andres@anarazel.de> wrote:
> > As a first step I'd be inclined to "just" adjust disable_cost up to
> > something like 1.0e12. Unfortunately much higher and and we're getting
> > into the area where the loss of precision starts to be significant
> > enough that I'm not sure that we're always careful enough to perform
> > math in the right order (e.g. 1.0e16 + 1 being 1.0e16, and 1e+20 + 1000
> > being 1e+20). I've seen queries with costs above 1e10 where that costing
> > wasn't insane.
> 
> We've done that before and we can do it again. But we're going to need
> to have something better eventually, I think, not just keep kicking
> the can down the road.

Yea, that's why I continued on to describe what we should do afterwards
;)


> Yet another approach would be to divide the cost into two parts, a
> "cost" component and a "violations" component. If two paths are
> compared, the one with fewer violations always wins; if it's a tie,
> they compare on cost. A path's violation count is the total of its
> children, plus one for itself if it does something that's disabled.
> This would be more principled than the current approach, but maybe
> it's too costly.

Namely go for something like this. I think we probably get away with the
additional comparison, especially if we were to store the violations as
an integer and did it like if (unlikely(path1->nviolations !=
path2->nviolations)) or such - that ought to be very well predicted in
nearly all cases.

I wonder how much we'd need to reformulate
compare_path_costs/compare_path_costs_fuzzily to allow the compiler to
auto-vectorize. Might not be worth caring...

Greetings,

Andres Freund

Zhenghua Lyu <zlv@pivotal.io> writes:
>> I think a more robust way to disable forbidden plan types would be to
>> handle the disabling in add_path(). Instead of having a high disable cost
>> on the Path itself, the comparison add_path() would always consider
>> disabled paths as more expensive than others, regardless of the cost.

Getting rid of disable_cost would be a nice thing to do, but I would
rather not do it by adding still more complexity to add_path(), not
to mention having to bloat Paths with a separate "disabled" marker.

The idea that I've been thinking about is to not generate disabled
Paths in the first place, thus not only fixing the problem but saving
some cycles.  While this seems easy enough for "optional" paths,
we have to reserve the ability to generate certain path types regardless,
if there's no other way to implement the query.  This is a bit of a
stumbling block :-(.  At the base relation level, we could do something
like generating seqscan last, and only if no other path has been
successfully generated.  But I'm not sure how to scale that up to
joins.  In particular, imagine that we consider joining A to B, and
find that the only way is a nestloop, so we generate a nestloop join
despite that being nominally disabled.  The next join level would
then see that as an available path, and it might decide that
((A nestjoin B) join C) is the cheapest choice, even though there
might have been a way to do, say, ((A join C) join B) with no use of
nestloops.  Users would find this surprising.

Maybe the only way to do this is a separate number-of-uses-of-
disabled-plan-types cost figure in Paths, but I still don't want
to go there.  The number of cases where disable_cost's shortcomings
really matter is too small to justify that, IMHO.

            regards, tom lane

Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> On Fri, Nov 01, 2019 at 09:30:52AM -0700, Jim Finnerty wrote:
>> re: coping with adding disable_cost more than once
>>
>> Another option would be to have a 2-part Cost structure.  If disable_cost is
>> ever added to the Cost, then you set a flag recording this.  If any plans
>> exist that have no disable_costs added to them, then the planner chooses the
>> minimum cost among those, otherwise you choose the minimum cost path.

> Yeah, I agree having is_disabled flag, and treat all paths with 'true'
> as more expensive than paths with 'false' (and when both paths have the
> same value then actually compare the cost) is probably the way forward.

It would have to be a count, not a boolean --- for example, you want to
prefer a path that uses one disabled SeqScan over a path that uses two.

I'm with Andres in being pretty worried about the extra burden imposed
on add_path comparisons.

            regards, tom lane

As a proof of concept, I hacked around a bit today to re-purpose one of the
bits of the Cost structure to mean "is_disabled" so that we can distinguish
'disabled' from 'non-disabled' paths without making the Cost structure any
bigger.  In fact, it's still a valid double.  The obvious choice would have
been to re-purpose the sign bit, but I've had occasion to exploit negative
costs before so for this POC I used the high-order bit of the fractional
bits of the double.  (see Wikipedia for double precision floating point for
the layout).

The idea is to set a special bit when disable_cost is added to a cost. 
Dedicating multiple bits instead of just 1 would be easily done, but as it
is we can accumulate many disable_costs without overflowing, so just
comparing the cost suffices.

The patch is not fully debugged and fails on a couple of tests in the serial
test suite.  It seems to fail on Cartesian products, and maybe in one other
non-CP case.  I wasn't able to debug it before the day came to an end.

In one place the core code subtracts off the disable_cost.  I left the
"disabled" bit set in this case, which might be wrong.

I don't see an option to attach the patch as an attachment, so here is the
patch inline (it is based on PG11).  The more interesting part is in a small
number of lines in costsize.c.  Other changes just add functions that assign
a disable_cost and set the bit, or that compare costs such that a
non-disabled cost always compares less than a disabled cost.

------------------

diff --git a/src/backend/optimizer/path/costsize.c
b/src/backend/optimizer/path/costsize.c
index 4e86458672..3718639330 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -123,6 +123,8 @@ double        parallel_setup_cost =
DEFAULT_PARALLEL_SETUP_COST;
 int            effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
 
 Cost        disable_cost = 1.0e10;
+uint64      disabled_mask = 0x8000000000000;
+#define IS_DISABLED(cost) (((uint64) cost) & disabled_mask)
 
 int            max_parallel_workers_per_gather = 2;
 
@@ -205,6 +207,53 @@ clamp_row_est(double nrows)
     return nrows;
 }
 
+Cost
+add_cost(Cost cost, Cost delta_cost)
+{
+    uint64 mask = (delta_cost == disable_cost) ? disabled_mask : 0;
+    Cost max_cost = disabled_mask - disable_cost;
+    
+    if (cost + delta_cost < max_cost)
+        return ((Cost) ((uint64)(cost + delta_cost) | mask));
+    else
+        return ((Cost) ((uint64)(max_cost) | mask));
+}
+
+bool
+is_lower_cost(Cost cost1, Cost cost2)
+{
+    if ((uint64)cost1 & disabled_mask && !((uint64)cost2 & disabled_mask))
+        return false;
+    
+    if (!((uint64)cost1 & disabled_mask) && (uint64)cost2 & disabled_mask)
+        return true;
+    
+    return (cost1 < cost2);
+}
+
+bool
+is_greater_cost(Cost cost1, Cost cost2)
+{
+    if ((uint64)cost1 & disabled_mask && !((uint64)cost2 & disabled_mask))
+        return true;
+    
+    if (!((uint64)cost1 & disabled_mask) && (uint64)cost2 & disabled_mask)
+        return false;
+    
+    return (cost1 > cost2);
+}
+
+bool
+is_geq_cost(Cost cost1, Cost cost2)
+{
+    if ((uint64)cost1 & disabled_mask && !((uint64)cost2 & disabled_mask))
+        return true;
+    
+    if (!((uint64)cost1 & disabled_mask) && (uint64)cost2 & disabled_mask)
+        return false;
+    
+    return (cost1 >= cost2);
+}
 
 /*
  * cost_seqscan
@@ -235,7 +284,7 @@ cost_seqscan(Path *path, PlannerInfo *root,
         path->rows = baserel->rows;
 
     if (!enable_seqscan)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     /* fetch estimated page cost for tablespace containing table */
     get_tablespace_page_costs(baserel->reltablespace,
@@ -424,7 +473,7 @@ cost_gather_merge(GatherMergePath *path, PlannerInfo
*root,
         path->path.rows = rel->rows;
 
     if (!enable_gathermerge)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     /*
      * Add one to the number of workers to account for the leader.  This might
@@ -538,7 +587,7 @@ cost_index(IndexPath *path, PlannerInfo *root, double
loop_count,
     }
 
     if (!enable_indexscan)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
     /* we don't need to check enable_indexonlyscan; indxpath.c does that */
 
     /*
@@ -976,7 +1025,7 @@ cost_bitmap_heap_scan(Path *path, PlannerInfo *root,
RelOptInfo *baserel,
         path->rows = baserel->rows;
 
     if (!enable_bitmapscan)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     pages_fetched = compute_bitmap_pages(root, baserel, bitmapqual,
                                          loop_count, &indexTotalCost,
@@ -1242,10 +1291,10 @@ cost_tidscan(Path *path, PlannerInfo *root,
     if (isCurrentOf)
     {
         Assert(baserel->baserestrictcost.startup >= disable_cost);
-        startup_cost -= disable_cost;
+        startup_cost -= disable_cost;  /* but do not un-set the disabled mark */
     }
     else if (!enable_tidscan)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     /*
      * The TID qual expressions will be computed once, any other baserestrict
@@ -1676,7 +1725,7 @@ cost_sort(Path *path, PlannerInfo *root,
     long        sort_mem_bytes = sort_mem * 1024L;
 
     if (!enable_sort)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     path->rows = tuples;
 
@@ -2121,8 +2170,8 @@ cost_agg(Path *path, PlannerInfo *root,
         total_cost = input_total_cost;
         if (aggstrategy == AGG_MIXED && !enable_hashagg)
         {
-            startup_cost += disable_cost;
-            total_cost += disable_cost;
+            startup_cost = add_cost(startup_cost, disable_cost);
+            total_cost = add_cost(total_cost, disable_cost);
         }
         /* calcs phrased this way to match HASHED case, see note above */
         total_cost += aggcosts->transCost.startup;
@@ -2137,7 +2186,7 @@ cost_agg(Path *path, PlannerInfo *root,
         /* must be AGG_HASHED */
         startup_cost = input_total_cost;
         if (!enable_hashagg)
-            startup_cost += disable_cost;
+            startup_cost = add_cost(startup_cost, disable_cost);
         startup_cost += aggcosts->transCost.startup;
         startup_cost += aggcosts->transCost.per_tuple * input_tuples;
         startup_cost += (cpu_operator_cost * numGroupCols) * input_tuples;
@@ -2436,7 +2485,7 @@ final_cost_nestloop(PlannerInfo *root, NestPath *path,
      * disabled, which doesn't seem like the way to bet.
      */
     if (!enable_nestloop)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     /* cost of inner-relation source data (we already dealt with outer rel) */
 
@@ -2882,7 +2931,7 @@ final_cost_mergejoin(PlannerInfo *root, MergePath
*path,
      * disabled, which doesn't seem like the way to bet.
      */
     if (!enable_mergejoin)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     /*
      * Compute cost of the mergequals and qpquals (other restriction clauses)
@@ -3312,7 +3361,7 @@ final_cost_hashjoin(PlannerInfo *root, HashPath *path,
      * disabled, which doesn't seem like the way to bet.
      */
     if (!enable_hashjoin)
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     /* mark the path with estimated # of batches */
     path->num_batches = numbatches;
@@ -3410,7 +3459,7 @@ final_cost_hashjoin(PlannerInfo *root, HashPath *path,
     if (relation_byte_size(clamp_row_est(inner_path_rows * innermcvfreq),
                            inner_path->pathtarget->width) >
         (work_mem * 1024L))
-        startup_cost += disable_cost;
+        startup_cost = add_cost(startup_cost, disable_cost);
 
     /*
      * Compute cost of the hashquals and qpquals (other restriction clauses)
@@ -3930,7 +3979,7 @@ cost_qual_eval_walker(Node *node,
cost_qual_eval_context *context)
     else if (IsA(node, CurrentOfExpr))
     {
         /* Report high cost to prevent selection of anything but TID scan */
-        context->total.startup += disable_cost;
+        context->total.startup = add_cost(context->total.startup, disable_cost);
     }
     else if (IsA(node, SubLink))
     {
diff --git a/src/backend/optimizer/util/pathnode.c
b/src/backend/optimizer/util/pathnode.c
index 4736d84a83..fd746a06bc 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -72,33 +72,33 @@ compare_path_costs(Path *path1, Path *path2,
CostSelector criterion)
 {
     if (criterion == STARTUP_COST)
     {
-        if (path1->startup_cost < path2->startup_cost)
+        if (is_lower_cost(path1->startup_cost, path2->startup_cost))
             return -1;
-        if (path1->startup_cost > path2->startup_cost)
+        if (is_greater_cost(path1->startup_cost, path2->startup_cost))
             return +1;
 
         /*
          * If paths have the same startup cost (not at all unlikely), order
          * them by total cost.
          */
-        if (path1->total_cost < path2->total_cost)
+        if (is_lower_cost(path1->total_cost, path2->total_cost))
             return -1;
-        if (path1->total_cost > path2->total_cost)
+        if (is_greater_cost(path1->total_cost, path2->total_cost))
             return +1;
     }
     else
     {
-        if (path1->total_cost < path2->total_cost)
+        if (is_lower_cost(path1->total_cost, path2->total_cost))
             return -1;
-        if (path1->total_cost > path2->total_cost)
+        if (is_greater_cost(path1->total_cost, path2->total_cost))
             return +1;
 
         /*
          * If paths have the same total cost, order them by startup cost.
          */
-        if (path1->startup_cost < path2->startup_cost)
+        if (is_lower_cost(path1->startup_cost, path2->startup_cost))
             return -1;
-        if (path1->startup_cost > path2->startup_cost)
+        if (is_greater_cost(path1->startup_cost, path2->startup_cost))
             return +1;
     }
     return 0;
@@ -126,9 +126,9 @@ compare_fractional_path_costs(Path *path1, Path *path2,
         fraction * (path1->total_cost - path1->startup_cost);
     cost2 = path2->startup_cost +
         fraction * (path2->total_cost - path2->startup_cost);
-    if (cost1 < cost2)
+    if (is_lower_cost(cost1, cost2))
         return -1;
-    if (cost1 > cost2)
+    if (is_greater_cost(cost1, cost2))
         return +1;
     return 0;
 }
@@ -172,11 +172,11 @@ compare_path_costs_fuzzily(Path *path1, Path *path2,
double fuzz_factor)
      * Check total cost first since it's more likely to be different; many
      * paths have zero startup cost.
      */
-    if (path1->total_cost > path2->total_cost * fuzz_factor)
+    if (is_greater_cost(path1->total_cost, path2->total_cost * fuzz_factor))
     {
         /* path1 fuzzily worse on total cost */
         if (CONSIDER_PATH_STARTUP_COST(path1) &&
-            path2->startup_cost > path1->startup_cost * fuzz_factor)
+            is_greater_cost(path2->startup_cost, path1->startup_cost * fuzz_factor))
         {
             /* ... but path2 fuzzily worse on startup, so DIFFERENT */
             return COSTS_DIFFERENT;
@@ -184,11 +184,11 @@ compare_path_costs_fuzzily(Path *path1, Path *path2,
double fuzz_factor)
         /* else path2 dominates */
         return COSTS_BETTER2;
     }
-    if (path2->total_cost > path1->total_cost * fuzz_factor)
+    if (is_greater_cost(path2->total_cost, path1->total_cost * fuzz_factor))
     {
         /* path2 fuzzily worse on total cost */
         if (CONSIDER_PATH_STARTUP_COST(path2) &&
-            path1->startup_cost > path2->startup_cost * fuzz_factor)
+            is_greater_cost(path1->startup_cost, path2->startup_cost * fuzz_factor))
         {
             /* ... but path1 fuzzily worse on startup, so DIFFERENT */
             return COSTS_DIFFERENT;
@@ -197,12 +197,12 @@ compare_path_costs_fuzzily(Path *path1, Path *path2,
double fuzz_factor)
         return COSTS_BETTER1;
     }
     /* fuzzily the same on total cost ... */
-    if (path1->startup_cost > path2->startup_cost * fuzz_factor)
+    if (is_greater_cost(path1->startup_cost, path2->startup_cost *
fuzz_factor))
     {
         /* ... but path1 fuzzily worse on startup, so path2 wins */
         return COSTS_BETTER2;
     }
-    if (path2->startup_cost > path1->startup_cost * fuzz_factor)
+    if (is_greater_cost(path2->startup_cost, path1->startup_cost *
fuzz_factor))
     {
         /* ... but path2 fuzzily worse on startup, so path1 wins */
         return COSTS_BETTER1;
@@ -605,7 +605,7 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
         else
         {
             /* new belongs after this old path if it has cost >= old's */
-            if (new_path->total_cost >= old_path->total_cost)
+            if (is_geq_cost(new_path->total_cost, old_path->total_cost))
                 insert_after = p1;
             /* p1_prev advances */
             p1_prev = p1;
@@ -681,7 +681,7 @@ add_path_precheck(RelOptInfo *parent_rel,
          *
          * Cost comparisons here should match compare_path_costs_fuzzily.
          */
-        if (total_cost > old_path->total_cost * STD_FUZZ_FACTOR)
+        if (is_greater_cost(total_cost, old_path->total_cost * STD_FUZZ_FACTOR))
         {
             /* new path can win on startup cost only if consider_startup */
             if (startup_cost > old_path->startup_cost * STD_FUZZ_FACTOR ||
@@ -796,14 +796,14 @@ add_partial_path(RelOptInfo *parent_rel, Path
*new_path)
         /* Unless pathkeys are incompable, keep just one of the two paths. */
         if (keyscmp != PATHKEYS_DIFFERENT)
         {
-            if (new_path->total_cost > old_path->total_cost * STD_FUZZ_FACTOR)
+            if (is_greater_cost(new_path->total_cost, old_path->total_cost *
STD_FUZZ_FACTOR))
             {
                 /* New path costs more; keep it only if pathkeys are better. */
                 if (keyscmp != PATHKEYS_BETTER1)
                     accept_new = false;
             }
-            else if (old_path->total_cost > new_path->total_cost
-                     * STD_FUZZ_FACTOR)
+            else if (is_greater_cost(old_path->total_cost, new_path->total_cost
+                                     * STD_FUZZ_FACTOR))
             {
                 /* Old path costs more; keep it only if pathkeys are better. */
                 if (keyscmp != PATHKEYS_BETTER2)
@@ -819,7 +819,7 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
                 /* Costs are about the same, old path has better pathkeys. */
                 accept_new = false;
             }
-            else if (old_path->total_cost > new_path->total_cost * 1.0000000001)
+            else if (is_greater_cost(old_path->total_cost, new_path->total_cost *
1.0000000001))
             {
                 /* Pathkeys are the same, and the old path costs more. */
                 remove_old = true;
@@ -847,7 +847,7 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
         else
         {
             /* new belongs after this old path if it has cost >= old's */
-            if (new_path->total_cost >= old_path->total_cost)
+            if (is_geq_cost(new_path->total_cost, old_path->total_cost))
                 insert_after = p1;
             /* p1_prev advances */
             p1_prev = p1;
@@ -913,10 +913,10 @@ add_partial_path_precheck(RelOptInfo *parent_rel, Cost
total_cost,
         keyscmp = compare_pathkeys(pathkeys, old_path->pathkeys);
         if (keyscmp != PATHKEYS_DIFFERENT)
         {
-            if (total_cost > old_path->total_cost * STD_FUZZ_FACTOR &&
+            if (is_greater_cost(total_cost, old_path->total_cost * STD_FUZZ_FACTOR)
&&
                 keyscmp != PATHKEYS_BETTER1)
                 return false;
-            if (old_path->total_cost > total_cost * STD_FUZZ_FACTOR &&
+            if (is_greater_cost(old_path->total_cost, total_cost * STD_FUZZ_FACTOR)
&&
                 keyscmp != PATHKEYS_BETTER2)
                 return true;
         }
@@ -1697,7 +1697,7 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel,
Path *subpath,
 
     if (sjinfo->semi_can_btree && sjinfo->semi_can_hash)
     {
-        if (agg_path.total_cost < sort_path.total_cost)
+        if (is_lower_cost(agg_path.total_cost, sort_path.total_cost))
             pathnode->umethod = UNIQUE_PATH_HASH;
         else
             pathnode->umethod = UNIQUE_PATH_SORT;
diff --git a/src/backend/utils/cache/relcache.c
b/src/backend/utils/cache/relcache.c
index 78f3b99a76..c261a9d790 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -5076,8 +5076,8 @@ IsProjectionFunctionalIndex(Relation index)
          * when values differ because the expression is recalculated when
          * inserting a new index entry for the changed value.
          */
-        if ((index_expr_cost.startup + index_expr_cost.per_tuple) >
-            HEURISTIC_MAX_HOT_RECHECK_EXPR_COST)
+        if (is_greater_cost((index_expr_cost.startup +
index_expr_cost.per_tuple),
+                            HEURISTIC_MAX_HOT_RECHECK_EXPR_COST))
             is_projection = false;
 
         tuple = SearchSysCache1(RELOID,
ObjectIdGetDatum(RelationGetRelid(index)));
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index 9159f2bab1..c01d08eae5 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -251,6 +251,12 @@ extern PathTarget
*set_pathtarget_cost_width(PlannerInfo *root, PathTarget *targ
 extern double compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel,
                      Path *bitmapqual, int loop_count, Cost *cost, double *tuple);
 
+extern Cost add_cost(Cost cost, Cost delta_cost);
+extern bool is_lower_cost(Cost cost1, Cost cost2);
+extern bool is_greater_cost(Cost cost1, Cost cost2);
+extern bool is_geq_cost(Cost cost1, Cost cost2);
+
+
 /*
  * prototypes for clausesel.c
  *      routines to compute clause selectivities





-----
Jim Finnerty, AWS, Amazon Aurora PostgreSQL
--
Sent from: https://www.postgresql-archive.org/PostgreSQL-hackers-f1928748.html

On Tue, 2019-12-10 at 15:50 -0700, Jim Finnerty wrote:
> As a proof of concept, I hacked around a bit today to re-purpose one of the
> bits of the Cost structure to mean "is_disabled" so that we can distinguish
> 'disabled' from 'non-disabled' paths without making the Cost structure any
> bigger.  In fact, it's still a valid double.  The obvious choice would have
> been to re-purpose the sign bit, but I've had occasion to exploit negative
> costs before so for this POC I used the high-order bit of the fractional
> bits of the double.  (see Wikipedia for double precision floating point for
> the layout).
> 
> The idea is to set a special bit when disable_cost is added to a cost. 
> Dedicating multiple bits instead of just 1 would be easily done, but as it
> is we can accumulate many disable_costs without overflowing, so just
> comparing the cost suffices.

Doesn't that rely on a specific implementation of double precision (IEEE)?
I thought that we don't want to limit ourselves to platforms with IEEE floats.

Yours,
Laurenz Albe

On Wed, 11 Dec 2019 at 01:24, Laurenz Albe <laurenz.albe@cybertec.at> wrote:
>
> On Tue, 2019-12-10 at 15:50 -0700, Jim Finnerty wrote:
> > As a proof of concept, I hacked around a bit today to re-purpose one of the
> > bits of the Cost structure to mean "is_disabled" so that we can distinguish
> > 'disabled' from 'non-disabled' paths without making the Cost structure any
> > bigger.  In fact, it's still a valid double.  The obvious choice would have
> > been to re-purpose the sign bit, but I've had occasion to exploit negative
> > costs before so for this POC I used the high-order bit of the fractional
> > bits of the double.  (see Wikipedia for double precision floating point for
> > the layout).
> >
> > The idea is to set a special bit when disable_cost is added to a cost.
> > Dedicating multiple bits instead of just 1 would be easily done, but as it
> > is we can accumulate many disable_costs without overflowing, so just
> > comparing the cost suffices.
>
> Doesn't that rely on a specific implementation of double precision (IEEE)?
> I thought that we don't want to limit ourselves to platforms with IEEE floats.

We could always implement it again in another format....

However, I wouldn't have expected to be bit twiddling. I would have
expected to use standard functions like ldexp to do this. In fact I
think if you use the high bit of the exponent you could do it entirely
using ldexp and regular double comparisons (with fabs).

Ie, to set the bit you set cost = ldexp(cost, __DBL_MAX_EXP__/2). And
to check for the bit being set you compare ilogb(cost,
__DBL_MAX_EXP__/2). Hm. that doesn't handle if the cost is already < 1
in which case I guess you would have to set it to 1 first. Or reserve
the two high bits of the cost so you can represent disabled values
that had negative exponents before being disabled.

I wonder if it wouldn't be a lot cleaner and more flexible to just go
with a plain float for Cost and use the other 32 bits for counters and
bitmasks and still be ahead of the game. A double can store 2^1024 but
a float 2^128 which still feels like it should be more than enough to
store the kinds of costs plans have without the disabled costs. 2^128
milliseconds is still 10^28 years which is an awfully expensive
query....

-- 
greg

On Wed, Dec 11, 2019 at 7:24 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:
> Doesn't that rely on a specific implementation of double precision (IEEE)?
> I thought that we don't want to limit ourselves to platforms with IEEE floats.

Just by the way, you might want to read the second last paragraph of
the commit message for 02ddd499.  The dream is over, we're never going
to run on Vax.

Thomas Munro <thomas.munro@gmail.com> writes:
> On Wed, Dec 11, 2019 at 7:24 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:
>> Doesn't that rely on a specific implementation of double precision (IEEE)?
>> I thought that we don't want to limit ourselves to platforms with IEEE floats.

> Just by the way, you might want to read the second last paragraph of
> the commit message for 02ddd499.  The dream is over, we're never going
> to run on Vax.

Still, the proposed hack is doubling down on IEEE dependency in a way
that I quite dislike, in that (a) it doesn't just read float values
but generates new ones (and assumes that the hardware/libc will react in
a predictable way to them), (b) in a part of the code that has no damn
business having close dependencies on float format, and (c) for a gain
far smaller than what we got from the Ryu code.

We have had prior discussions about whether 02ddd499 justifies adding
more IEEE dependencies elsewhere.  I don't think it does.  IEEE 754
is not the last word that will ever be said on floating-point arithmetic,
any more than x86_64 is the last CPU architecture that anyone will ever
care about.  We should keep our dependencies on it well circumscribed.

            regards, tom lane

On Thu, Aug 3, 2023 at 5:22 AM Jian Guo <gjian@vmware.com> wrote:
> I have write an initial patch to retire the disable_cost GUC, which labeled a flag on the Path struct instead of
addingup a big cost which is hard to estimate. Though it involved in tons of plan changes in regression tests, I have
testedon some simple test cases such as eagerly generate a two-stage agg plans and it worked. Could someone help to
review?

I took a look at this patch today. I believe that overall this may
well be an approach worth pursuing. However, more work is going to be
needed. Here are some comments:

1. You stated that it changes lots of plans in the regression tests,
but you haven't provided any sort of analysis of why those plans
changed. I'm kind of surprised that there would be "tons" of plan
changes. You (or someone) should look into why that's happening.

2. The change to compare_path_costs_fuzzily() seems incorrect to me.
When path1->is_disabled && path2->is_disabled, costs should be
compared just as they are when neither path is disabled. Instead, the
patch treats any two such paths as having equal cost. That seems
catastrophically bad. Maybe it accounts for some of those plan
changes, although that would only be true if those plans were created
while using some disabling GUC.

3. Instead of adding is_disabled at the end of the Path structure, I
suggest adding it between param_info and parallel_aware. I think if
you do that, the space used by the new field will use up padding bytes
that are currently included in the struct, instead of making it
longer.

4. A critical issue for any patch of this type is performance. This
concern was raised earlier on this thread, but your path doesn't
address it. There's no performance analysis or benchmarking included
in your email. One idea that I have is to write the cost-comparison
test like this:

if (unlikely(path1->is_disabled || path2->is_disabled))
{
    if (!path1->is_disabled)
        return COSTS_BETTER1;
    if (!path2->is_disabled)
        return COSTS_BETTER2;
    /* if both disabled, fall through */
}

I'm not sure that would be enough to prevent the patch from adding
noticeably to the cost of path comparison, but maybe it would help.

5. The patch changes only compare_path_costs_fuzzily() but I wonder
whether compare_path_costs() and compare_fractional_path_costs() need
similar surgery. Whether they do or don't, there should likely be some
comments explaining the situation.

6. In fact, the patch changes no comments at all, anywhere. I'm not
sure how many comment changes a patch like this needs to make, but the
answer definitely isn't "none".

7. The patch doesn't actually remove disable_cost. I guess it should.

8. When you submit a patch, it's a good idea to also add it on
commitfest.postgresql.org. It doesn't look like that was done in this
case.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Aug 3, 2023 at 5:22 AM Jian Guo <gjian@vmware.com> wrote:
>> I have write an initial patch to retire the disable_cost GUC, which labeled a flag on the Path struct instead of
addingup a big cost which is hard to estimate. Though it involved in tons of plan changes in regression tests, I have
testedon some simple test cases such as eagerly generate a two-stage agg plans and it worked. Could someone help to
review?

> I took a look at this patch today. I believe that overall this may
> well be an approach worth pursuing. However, more work is going to be
> needed. Here are some comments:

> 1. You stated that it changes lots of plans in the regression tests,
> but you haven't provided any sort of analysis of why those plans
> changed. I'm kind of surprised that there would be "tons" of plan
> changes. You (or someone) should look into why that's happening.

I've not read the patch, but given this description I would expect
there to be *zero* regression changes --- I don't think we have any
test cases that depend on disable_cost being finite.  If there's more
than zero changes, that very likely indicates a bug in the patch.
Even if there are places where the output legitimately changes, you
need to justify each one and make sure that you didn't invalidate the
intent of that test case.

BTW, having written that paragraph, I wonder if we couldn't get
the same end result with a nearly one-line change that consists of
making disable_cost be IEEE infinity.  Years ago we didn't want
to rely on IEEE float semantics in this area, but nowadays I don't
see why we shouldn't.

            regards, tom lane

On Tue, Mar 12, 2024 at 1:32 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> BTW, having written that paragraph, I wonder if we couldn't get
> the same end result with a nearly one-line change that consists of
> making disable_cost be IEEE infinity.  Years ago we didn't want
> to rely on IEEE float semantics in this area, but nowadays I don't
> see why we shouldn't.

I don't think so, because I think that what will happen in that case
is that we'll pick a completely random plan if we can't pick a plan
that avoids incurring disable_cost. Every plan that contains one
disabled node anywhere in the plan tree will look like it has exactly
the same cost as any other such plan.

IMHO, this is actually one of the problems with disable_cost as it
works today. I think the semantics that we want are: if it's possible
to pick a plan where nothing is disabled, then pick the cheapest such
plan; if not, pick the cheapest plan overall. But treating
disable_cost doesn't really do that. It does the first part -- picking
the cheapest plan where nothing is disabled -- but it doesn't do the
second part, because once you add disable_cost into the cost of some
particular plan node, it screws up the rest of the planning, because
the cost estimates for the disabled nodes have no bearing in reality.
Fast-start plans, for example, will look insanely good compared to
what would be the case in normal planning (and we lean too much toward
fast-start plans even normally).

(I don't think we should care how MANY disabled nodes appear in a
plan, particularly. This is a more arguable point. Is a plan with 1
disabled node and 10% more cost better or worse than a plan with 2
disabled nodes and 10% less cost? I'd argue that counting the number
of disabled nodes isn't particularly meaningful.)

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Mar 12, 2024 at 1:32 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> BTW, having written that paragraph, I wonder if we couldn't get
>> the same end result with a nearly one-line change that consists of
>> making disable_cost be IEEE infinity.

> I don't think so, because I think that what will happen in that case
> is that we'll pick a completely random plan if we can't pick a plan
> that avoids incurring disable_cost. Every plan that contains one
> disabled node anywhere in the plan tree will look like it has exactly
> the same cost as any other such plan.

Good point.

> IMHO, this is actually one of the problems with disable_cost as it
> works today.

Yeah.  I keep thinking that the right solution is to not generate
disabled paths in the first place if there are any other ways to
produce the same relation.  That has obvious order-of-operations
problems though, and I've not been able to make it work.

            regards, tom lane

On Tue, Mar 12, 2024 at 3:36 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Yeah.  I keep thinking that the right solution is to not generate
> disabled paths in the first place if there are any other ways to
> produce the same relation.  That has obvious order-of-operations
> problems though, and I've not been able to make it work.

I've expressed the same view in the past. It would be nice not to
waste planner effort on paths that we're just going to throw away, but
I'm not entirely sure what you mean by "obvious order-of-operations
problems."

To me, it seems like what we'd need is to be able to restart the whole
planner process if we run out of steam before we get done. For
example, suppose we're planning a 2-way join where index and
index-only scans are disabled, sorts are disabled, and nested loops
and hash joins are disabled. There's no problem generating just the
non-disabled scan types at the baserel level, but when we reach the
join, we're going to find that the only non-disabled join type is a
merge join, and we're also going to find that we have no paths that
provide pre-sorted input, so we need to sort, which we're also not
allowed to do. If we could give up at that point and restart planning,
disabling all of the plan-choice constraints and now creating all
paths for each RelOptInfo, then everything would, I believe, be just
fine. We'd end up needing neither disable_cost nor the mechanism
proposed by this patch.

But in the absence of that, we need some way to privilege the
non-disabled paths over the disabled ones -- and I'd prefer to have
something more principled than disable_cost, if we can work out the
details.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Wed, 13 Mar 2024 at 08:55, Robert Haas <robertmhaas@gmail.com> wrote:
> But in the absence of that, we need some way to privilege the
> non-disabled paths over the disabled ones -- and I'd prefer to have
> something more principled than disable_cost, if we can work out the
> details.

The primary place I see issues with disabled_cost is caused by
STD_FUZZ_FACTOR.  When you add 1.0e10 to a couple of modestly costly
paths, it makes them appear fuzzily the same in cases where one could
be significantly cheaper than the other. If we were to bump up the
disable_cost it would make this problem worse.

I think we do still need some way to pick the cheapest disabled path
when there are no other options.

One way would be to set fuzz_factor to 1.0 when either of the paths
costs in compare_path_costs_fuzzily() is >= disable_cost.
clamp_row_est() does cap row estimates at MAXIMUM_ROWCOUNT (1e100), so
I think there is some value of disable_cost that could almost
certainly ensure we don't reach it because the path is truly expensive
rather than disabled.

So maybe the fix could be to set disable_cost to something like
1.0e110 and adjust compare_path_costs_fuzzily to not apply the
fuzz_factor for paths >= disable_cost.   However, I wonder if that
risks the costs going infinite after a couple of cartesian joins.

David

David Rowley <dgrowleyml@gmail.com> writes:
> So maybe the fix could be to set disable_cost to something like
> 1.0e110 and adjust compare_path_costs_fuzzily to not apply the
> fuzz_factor for paths >= disable_cost.   However, I wonder if that
> risks the costs going infinite after a couple of cartesian joins.

Perhaps.  It still does nothing for Robert's point that once we're
forced into using a "disabled" plan type, it'd be better if the
disabled-ness didn't skew subsequent planning choices.

On the whole I agree that getting rid of disable_cost entirely
would be the way to go, if we can replace that with a separate
boolean without driving up the cost of add_path too much.

            regards, tom lane

On Tue, Mar 12, 2024 at 4:55 PM David Rowley <dgrowleyml@gmail.com> wrote:
> The primary place I see issues with disabled_cost is caused by
> STD_FUZZ_FACTOR.  When you add 1.0e10 to a couple of modestly costly
> paths, it makes them appear fuzzily the same in cases where one could
> be significantly cheaper than the other. If we were to bump up the
> disable_cost it would make this problem worse.

Hmm, good point.

> So maybe the fix could be to set disable_cost to something like
> 1.0e110 and adjust compare_path_costs_fuzzily to not apply the
> fuzz_factor for paths >= disable_cost.   However, I wonder if that
> risks the costs going infinite after a couple of cartesian joins.

Yeah, I think the disabled flag is a better answer if we can make it
work. No matter what value we pick for disable_cost, it's bound to
skew the planning sometimes.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Tue, Mar 12, 2024 at 1:32 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > 1. You stated that it changes lots of plans in the regression tests,
> > but you haven't provided any sort of analysis of why those plans
> > changed. I'm kind of surprised that there would be "tons" of plan
> > changes. You (or someone) should look into why that's happening.
>
> I've not read the patch, but given this description I would expect
> there to be *zero* regression changes --- I don't think we have any
> test cases that depend on disable_cost being finite.  If there's more
> than zero changes, that very likely indicates a bug in the patch.
> Even if there are places where the output legitimately changes, you
> need to justify each one and make sure that you didn't invalidate the
> intent of that test case.

I spent some more time poking at this patch. It's missing a ton of
important stuff and is wrong in a whole bunch of really serious ways,
and I'm not going to try to mention all of them in this email. But I
do want to talk about some of the more interesting realizations that
came to me as I was working my way through this.

One of the things I realized relatively early is that the patch does
nothing to propagate disable_cost upward through the plan tree. That
means that if you have a choice between, say,
Sort-over-Append-over-SeqScan and MergeAppend-over-IndexScan, as we do
in the regression tests, disabling IndexScan doesn't change the plan
with the patch applied, as it does in master. That's because only the
IndexScan node ends up flagged as disabled. Once we start stacking
other plan nodes on top of disabled plan nodes, the resultant plans
are at no disadvantage compared to plans containing no disabled nodes.
The IndexScan plan survives initially, despite being disabled, because
it's got a sort order. That lets us use it to build a MergeAppend
path, and that MergeAppend path is not disabled, and compares
favorably on cost.

After straining my brain over various plan changes for a long time,
and hacking on the code somewhat, I realized that just propagating the
Boolean upward is insufficient to set things right. That's basically
because I was being dumb when I said this:

> I don't think we should care how MANY disabled nodes appear in a
> plan, particularly.

Suppose we try to plan a Nested Loop with SeqScan disabled, but
there's no alternative to a SeqScan for the outer side of the join. If
we suppose an upward-propagating Boolean, every path for the join is
disabled because every path for the outer side is disabled. That means
that we have no reason to avoid paths where the inner side also uses a
disabled path. When we loop over the inner rel's pathlist looking for
ways to build a path for the join, we may find some disabled paths
there, and the join paths we build from those paths are disabled, but
so are the join paths where we use a non-disabled path on the inner
side. So those paths are just competing with each other on cost, and
the path type that is supposedly disabled on the outer side of the
join ends up not really being very disabled at all. More precisely, if
disabling a plan type causes paths to be discarded completely before
the join paths are constructed, then they actually do get removed from
consideration. But if those paths make it into inner rel's path list,
even way out towards the end, then paths derived from them can jump to
the front of the joinrel's path list.

The same kind of problem happens with Append or MergeAppend nodes. The
regression tests expect that we'll avoid disabled plan types whenever
possible even if we can't avoid them completely; for instance, the
matest0 table intentionally omits an index on one child table.
Disabling sequential scans is expected to disable them for all of the
other child tables even though for that particular child table there
is no other option. But that behavior is hard to achieve if every path
for the parent rel is "equally disabled". You either want the path
that uses only the one required SeqScan to be not-disabled even though
one of its children is disabled ... or you want the disabled flag to
be more than a Boolean. And while there's probably more than one way
to make it work, the easiest thing seems to be to just have a
disabled-counter in every node that gets initialized to the total
disabled-counter values of all of its children, and then you add 1 if
that node is itself doing something that is disabled, i.e. the exact
opposite of what I said in the quote above.

I haven't done enough work to know whether that would match the
current behavior, let alone whether it would have acceptable
performance, and I'm not at all convinced that's the right direction
anyway. Actually, at the moment, I don't have a very good idea at all
what the right direction is. I do have a feeling that this patch is
probably not going in the right direction, but I might be wrong about
that, too.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> One of the things I realized relatively early is that the patch does
> nothing to propagate disable_cost upward through the plan tree.
> ...
> After straining my brain over various plan changes for a long time,
> and hacking on the code somewhat, I realized that just propagating the
> Boolean upward is insufficient to set things right. That's basically
> because I was being dumb when I said this:
>> I don't think we should care how MANY disabled nodes appear in a
>> plan, particularly.

Very interesting, thanks for the summary.  So the fact that
disable_cost is additive across plan nodes is actually a pretty
important property of the current setup.  I think this is closely
related to one argument you made against my upthread idea of using
IEEE Infinity for disable_cost: that'd mask whether more than one
of the sub-plans had been disabled.

> ... And while there's probably more than one way
> to make it work, the easiest thing seems to be to just have a
> disabled-counter in every node that gets initialized to the total
> disabled-counter values of all of its children, and then you add 1 if
> that node is itself doing something that is disabled, i.e. the exact
> opposite of what I said in the quote above.

Yeah, that seems like the next thing to try if anyone plans to pursue
this further.  That'd essentially do what we're doing now except that
disable_cost is its own "order of infinity", entirely separate from
normal costs.

            regards, tom lane

On Mon, Apr 1, 2024 at 5:00 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Very interesting, thanks for the summary.  So the fact that
> disable_cost is additive across plan nodes is actually a pretty
> important property of the current setup.  I think this is closely
> related to one argument you made against my upthread idea of using
> IEEE Infinity for disable_cost: that'd mask whether more than one
> of the sub-plans had been disabled.

Yes, exactly. I just hadn't quite put the pieces together.

> Yeah, that seems like the next thing to try if anyone plans to pursue
> this further.  That'd essentially do what we're doing now except that
> disable_cost is its own "order of infinity", entirely separate from
> normal costs.

Right. I think that's actually what I had in mind in the last
paragraph of http://postgr.es/m/CA+TgmoY+Ltw7B=1FSFSN4yHcu2roWrz-ijBovj-99LZU=9h1dA@mail.gmail.com
but that was a while ago and I'd lost track of why it actually
mattered. But I also have questions about whether that's really the
right approach.

I think the approach of just not generating paths we don't want in the
first place merits more consideration. We do that in some cases
already, but not in others, and I'm not clear why. Like, if
index-scans, index-only scans, sorts, nested loops, and hash joins are
disabled, something is going to have to give, because the only
remaining join type is a merge join yet we've ruled out every possible
way of getting the day into some order, but I'm not sure whether
there's some reason that we need exactly the behavior that we have
right now rather than anything else. Maybe it would be OK to just
insist on at least one unparameterized, non-partial path at the
baserel level, and then if that ends up forcing us to ignore the
join-type restrictions higher up, so be it. Or maybe that's not OK and
after I try that out I'll end up writing another email about how I was
a bit clueless about all of this. I don't know. But I feel like it
merits more investigation, because I'm having trouble shaking the
theory that what we've got right now is pretty arbitrary.

And also ... looking at the regression tests, and also thinking about
the kinds of problems that I think people run into in real
deployments, I can't help feeling like we've somehow got this whole
thing backwards. enable_wunk imagines that you want to plan as normal
except with one particular plan type excluded from consideration. And
maybe that makes sense if the point of the enable_wunk GUC is that the
planner feature might be buggy and you might therefore want to turn it
off to protect yourself, or if the planner feature might be expensive
and you might want to turn it off to save cycles. But surely that's
not the case with something like enable_seqscan or enable_indexscan.
What I think we're mostly doing in the regression tests is shutting
off every relevant type of plan except one. I theorize that what we
actually want to do is tell the planner what we do want to happen,
rather than what we don't want to happen, but we've got this weird set
of GUCs that do the opposite of that and we're super-attached to them
because they've existed forever. I don't really have a concrete
proposal here, but I wonder if what we're actually talking about here
is spending time and energy polishing a mechanism that nobody likes in
the first place.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Tue, Apr 2, 2024 at 10:04 AM Greg Sabino Mullane <htamfids@gmail.com> wrote:
> So rather than listing all the things we don't want to happen, we need a way to force (nay, highly encourage) a
particularsolution. As our costing is a based on positive numbers, what if we did something like this in costsize.c? 
>
>  Cost        disable_cost = 1.0e10;
>  Cost        promotion_cost = 1.0e10; // or higher or lower, depending on how strongly we want to "beat"
disable_costseffects. 
> ...
>
>     if (!enable_seqscan)
>         startup_cost += disable_cost;
>     else if (promote_seqscan)
>         startup_cost -= promotion_cost; // or replace "promote" with "encourage"?

I'm pretty sure negative costs are going to create a variety of
unpleasant planning artifacts. The large positive costs do, too, which
is where this whole discussion started. If I disable (or promote) some
particular plan, I want the rest of the plan tree to come out looking
as much as possible like what would have happened if the same
alternative had won organically on cost. I think the only reason we're
driving this off of costing today is that making add_path() more
complicated is unappealing, mostly on performance grounds, and if you
add disabled-ness (or promoted-ness) as a separate axis of value then
add_path() has to know about that on top of everything else. I think
the goal here is to come up with a more principled alternative that
isn't just based on whacking large numbers into the cost and hoping
something good happens ... but it is a whole lot easier to be unhappy
with the status quo than it is to come up with something that's
actually better.

I am planning to spend some more time thinking about it, though.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Apr 2, 2024 at 10:04 AM Greg Sabino Mullane <htamfids@gmail.com> wrote:
>> if (!enable_seqscan)
>>     startup_cost += disable_cost;
>> else if (promote_seqscan)
>>     startup_cost -= promotion_cost; // or replace "promote" with "encourage"?

> I'm pretty sure negative costs are going to create a variety of
> unpleasant planning artifacts.

Indeed.  It might be okay to have negative values for disabled-ness
if we treat disabled-ness as a "separate order of infinity", but
I suspect that it'd behave poorly when there are both disabled and
promoted sub-paths in a tree, for pretty much the same reasons you
explained just upthread.

> I think the only reason we're
> driving this off of costing today is that making add_path() more
> complicated is unappealing, mostly on performance grounds, and if you
> add disabled-ness (or promoted-ness) as a separate axis of value then
> add_path() has to know about that on top of everything else.

It doesn't seem to me that it's a separate axis of value, just a
higher-order component of the cost metric.  Nonetheless, adding even
a few instructions to add_path comparisons sounds expensive.  Maybe
it'd be fine, but we'd need to do some performance testing.

            regards, tom lane

On Tue, Apr 2, 2024 at 11:54 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > I'm pretty sure negative costs are going to create a variety of
> > unpleasant planning artifacts.
>
> Indeed.  It might be okay to have negative values for disabled-ness
> if we treat disabled-ness as a "separate order of infinity", but
> I suspect that it'd behave poorly when there are both disabled and
> promoted sub-paths in a tree, for pretty much the same reasons you
> explained just upthread.

Hmm, can you explain further? I think essentially you'd be maximizing
#(promoted notes)-#(disabled nodes), but I have no real idea whether
that behavior will be exactly what people want or extremely
unintuitive or something in the middle. It seems like it should be
fine if there's only promoting or only disabling or if we can respect
both the promoting and the disabling, assuming we even want to have
both, but I'm suspicious that it will be weird somehow in other cases.
I can't say exactly in what way, though. Do you have more insight?

> > I think the only reason we're
> > driving this off of costing today is that making add_path() more
> > complicated is unappealing, mostly on performance grounds, and if you
> > add disabled-ness (or promoted-ness) as a separate axis of value then
> > add_path() has to know about that on top of everything else.
>
> It doesn't seem to me that it's a separate axis of value, just a
> higher-order component of the cost metric.  Nonetheless, adding even
> a few instructions to add_path comparisons sounds expensive.  Maybe
> it'd be fine, but we'd need to do some performance testing.

Hmm, yeah. I'm not sure how much difference there is between these
things in practice. I didn't run down everything that was happening,
but I think what I did was equivalent to making it a higher-order
component of the cost metric, and it seemed like an awful lot of paths
were surviving anyway, e.g. index scans survived
enable_indexscan=false because they had a sort order, and I think
sequential scans were surviving enable_seqscan=false too, perhaps
because they had no startup cost. At any rate there's no question that
add_path() is hot.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Apr 2, 2024 at 11:54 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I suspect that it'd behave poorly when there are both disabled and
>> promoted sub-paths in a tree, for pretty much the same reasons you
>> explained just upthread.

> Hmm, can you explain further? I think essentially you'd be maximizing
> #(promoted notes)-#(disabled nodes), but I have no real idea whether
> that behavior will be exactly what people want or extremely
> unintuitive or something in the middle. It seems like it should be
> fine if there's only promoting or only disabling or if we can respect
> both the promoting and the disabling, assuming we even want to have
> both, but I'm suspicious that it will be weird somehow in other cases.
> I can't say exactly in what way, though. Do you have more insight?

Not really.  But if you had, say, a join of a promoted path to a
disabled path, that would be treated as on-par with a join of two
regular paths, which seems like it'd lead to odd choices.  Maybe
it'd be fine, but my gut says it'd likely not act nicely.  As you
say, it's a lot easier to believe that only-promoted or only-disabled
situations would behave sanely.

            regards, tom lane

On Tue, Apr 2, 2024 at 12:58 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Not really.  But if you had, say, a join of a promoted path to a
> disabled path, that would be treated as on-par with a join of two
> regular paths, which seems like it'd lead to odd choices.  Maybe
> it'd be fine, but my gut says it'd likely not act nicely.  As you
> say, it's a lot easier to believe that only-promoted or only-disabled
> situations would behave sanely.

Makes sense.

I wanted to further explore the idea of just not generating plans of
types that are currently disabled. I looked into doing this for
enable_indexscan and enable_indexonlyscan. As a first step, I
investigated how those settings work now, and was horrified. I don't
know whether I just wasn't paying attention back when the original
index-only scan work was done -- I remember discussing
enable_indexonlyscan with you at the time -- or whether it got changed
subsequently. Anyway, the current behavior is:

[A] enable_indexscan=false adds disable_cost to the cost of every
Index Scan path *and also* every Index-Only Scan path. So disabling
index-scans also in effect discourages the use of index-only scans,
which would make sense if we didn't have a separate setting called
enable_indexonlyscan, but we do. Given that, I think this is
completely and utterly wrong.

[b] enable_indexonlyscan=false causes index-only scan paths not to be
generated at all, but instead, we generate index-scan paths to do the
same thing that we would not have generated otherwise. This is weird
because it means that disabling one plan type causes us to consider
additional plans of another type, which seems like a thing that a user
might not expect. It's more defensible than [A], though, because you
could argue that we only omit the index scan path as an optimization,
on the theory that it will always lose to the index-only scan path,
and thus if the index-only scan path is not generated, there's a point
to generating the index scan path after all, so we should. However, it
seems unlikely to me that someone reading the one line of
documentation that we have about this parameter would be able to guess
that it works this way.

Here's an example of how the current system behaves:

robert.haas=# explain select count(*) from pgbench_accounts;
                                                   QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=2854.29..2854.30 rows=1 width=8)
   ->  Index Only Scan using pgbench_accounts_pkey on pgbench_accounts
 (cost=0.29..2604.29 rows=100000 width=0)
(2 rows)

robert.haas=# set enable_indexscan=false;
SET
robert.haas=# explain select count(*) from pgbench_accounts;
                                  QUERY PLAN
------------------------------------------------------------------------------
 Aggregate  (cost=2890.00..2890.01 rows=1 width=8)
   ->  Seq Scan on pgbench_accounts  (cost=0.00..2640.00 rows=100000 width=0)
(2 rows)

robert.haas=# set enable_seqscan=false;
SET
robert.haas=# set enable_bitmapscan=false;
SET
robert.haas=# explain select count(*) from pgbench_accounts;
                                                            QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=10000002854.29..10000002854.30 rows=1 width=8)
   ->  Index Only Scan using pgbench_accounts_pkey on pgbench_accounts
 (cost=10000000000.29..10000002604.29 rows=100000 width=0)
(2 rows)

robert.haas=# set enable_indexonlyscan=false;
SET
robert.haas=# explain select count(*) from pgbench_accounts;
                                          QUERY PLAN
-----------------------------------------------------------------------------------------------
 Aggregate  (cost=10000002890.00..10000002890.01 rows=1 width=8)
   ->  Seq Scan on pgbench_accounts
(cost=10000000000.00..10000002640.00 rows=100000 width=0)
(2 rows)

The first time we run the query, it picks an index-only scan because
it's the cheapest. When index scans are disabled, the query now picks
a sequential scan, even though it wasn't use an index-only scan,
because the index scan that it was using is perceived to have become
very expensive. When we then shut off sequential scans and bitmap-only
scans, it switches back to an index-only scan, because setting
enable_indexscan=false didn't completely disable index-only scans, but
just made them expensive. But now everything looks expensive, so we go
back to the same plan we had initially, except with the cost increased
by a bazillion. Finally, when we disable index-only scans, that
removes that plan from the pool, so now we pick the second-cheapest
plan overall, which in this case is a sequential scan.

So just to see what would happen, I wrote a patch to make
enable_indexscan and enable_indexonlyscan do exactly what they say on
the tin: when you set one of them to false, paths of that type are not
generated, and nothing else changes. I found that there are a
surprisingly large number of regression tests that rely on the current
behavior, so I took a crack at fixing them to achieve their goals (or
what I believed their goals to be) in other ways. The resulting patch
is attached for your (or anyone's) possible edification.

Just to be clear, I have no immediate plans to press forward with
trying to get something committed here. It seems pretty clear to me
that we should fix [A] in some way, but maybe not in the way I did it
here. It's also pretty clear to me that the fact that enable_indexscan
and enable_indexonlyscan work completely differently from each other
is surprising at best, wrong at worst, but here again, what this patch
does about that is not above reproach. I think it may make sense to
dig through the behavior of some of the remaining enable_* GUCs before
settling on a final strategy here, but I thought that the finds above
were interesting enough and bizarre enough that it made sense to drop
an email now and see what people think of all this before going
further.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Thu, 4 Apr 2024 at 08:21, Robert Haas <robertmhaas@gmail.com> wrote:
> I wanted to further explore the idea of just not generating plans of
> types that are currently disabled. I looked into doing this for
> enable_indexscan and enable_indexonlyscan. As a first step, I
> investigated how those settings work now, and was horrified. I don't
> know whether I just wasn't paying attention back when the original
> index-only scan work was done -- I remember discussing
> enable_indexonlyscan with you at the time -- or whether it got changed
> subsequently. Anyway, the current behavior is:
>
> [A] enable_indexscan=false adds disable_cost to the cost of every
> Index Scan path *and also* every Index-Only Scan path. So disabling
> index-scans also in effect discourages the use of index-only scans,
> which would make sense if we didn't have a separate setting called
> enable_indexonlyscan, but we do. Given that, I think this is
> completely and utterly wrong.
>
> [b] enable_indexonlyscan=false causes index-only scan paths not to be
> generated at all, but instead, we generate index-scan paths to do the
> same thing that we would not have generated otherwise.

FWIW, I think changing this is a bad idea and I don't think the
behaviour that's in your patch is useful.  With your patch, if I SET
enable_indexonlyscan=false, any index that *can* support an IOS for my
query will now not be considered at all!

With your patch applied, I see:

-- default enable_* GUC values.
postgres=# explain select oid from pg_class order by oid;
                                        QUERY PLAN
-------------------------------------------------------------------------------------------
 Index Only Scan using pg_class_oid_index on pg_class
(cost=0.27..22.50 rows=415 width=4)
(1 row)


postgres=# set enable_indexonlyscan=0; -- no index scan?
SET
postgres=# explain select oid from pg_class order by oid;
                           QUERY PLAN
-----------------------------------------------------------------
 Sort  (cost=36.20..37.23 rows=415 width=4)
   Sort Key: oid
   ->  Seq Scan on pg_class  (cost=0.00..18.15 rows=415 width=4)
(3 rows)

postgres=# set enable_seqscan=0; -- still no index scan!
SET
postgres=# explain select oid from pg_class order by oid;
                                     QUERY PLAN
------------------------------------------------------------------------------------
 Sort  (cost=10000000036.20..10000000037.23 rows=415 width=4)
   Sort Key: oid
   ->  Seq Scan on pg_class  (cost=10000000000.00..10000000018.15
rows=415 width=4)
(3 rows)

postgres=# explain select oid from pg_class order by oid,relname; --
now an index scan?!
                                         QUERY PLAN
---------------------------------------------------------------------------------------------
 Incremental Sort  (cost=0.43..79.50 rows=415 width=68)
   Sort Key: oid, relname
   Presorted Key: oid
   ->  Index Scan using pg_class_oid_index on pg_class
(cost=0.27..60.82 rows=415 width=68)
(4 rows)

I don't think this is good as pg_class_oid_index effectively won't be
used as long as the particular query could use that index with an
index *only* scan. You can see above that as soon as I adjust the
query slightly so that IOS isn't possible, the index can be used
again. I think an Index Scan would have been a much better option for
the 2nd query than the seq scan and sort.

I think if I do SET enable_indexonlyscan=0; the index should still be
used with an Index Scan and it definitely shouldn't result in Index
Scan also being disabled if that index happens to contain all the
columns required to support an IOS.

FWIW, I'm fine with the current behaviour.  It looks like we've
assumed that, when possible, IOS are always superior to Index Scan, so
there's no point in generating an Index Scan path when we can generate
an IOS path. I think this makes sense. For that not to be true,
checking the all visible flag would have to be more costly than
visiting the heap.  Perhaps that could be true if the visibility map
page had to come from disk and the heap page was cached and the disk
was slow, but I don't think that scenario is worthy of considering
both Index scan and IOS path types when IOS is possible. We've no way
to accurately cost that anyway.

This all seems similar to enable_sort vs enable_incremental_sort. For
a while, we did consider both an incremental sort and a sort when an
incremental sort was possible, but it seemed to me that an incremental
sort would always be better when it was possible, so I changed that in
4a29eabd1. I've not seen anyone complain.  I made it so that when an
incremental sort is possible but is disabled, we do a sort instead.
That seems fairly similar to how master handles
enable_indexonlyscan=false.

In short, I don't find it strange that disabling one node type results
in considering another type that we'd otherwise not consider in cases
where we assume that the disabled node type is always superior and
should always be used when it is possible.

I do agree that adding disable_cost to IOS when enable_indexscan=0 is
a bit weird.  We don't penalise incremental sorts when sorts are
disabled, so aligning those might make sense.

David

On Thu, 4 Apr 2024 at 10:15, David Rowley <dgrowleyml@gmail.com> wrote:
> In short, I don't find it strange that disabling one node type results
> in considering another type that we'd otherwise not consider in cases
> where we assume that the disabled node type is always superior and
> should always be used when it is possible.

In addition to what I said earlier, I think the current
enable_indexonlyscan is implemented in a way that has the planner do
what it did before IOS was added.  I think that goal makes sense with
any patch that make the planner try something new. We want to have
some method to get the previous behaviour for the cases where the
planner makes a dumb choice or to avoid some bug in the new feature.

I think using that logic, the current scenario with enable_indexscan
and enable_indexonlyscan makes complete sense. I mean, including
enable_indexscan=0 adding disable_cost to IOS Paths.

David

On Wed, Apr 3, 2024 at 11:09 PM David Rowley <dgrowleyml@gmail.com> wrote:
> On Thu, 4 Apr 2024 at 10:15, David Rowley <dgrowleyml@gmail.com> wrote:
> > In short, I don't find it strange that disabling one node type results
> > in considering another type that we'd otherwise not consider in cases
> > where we assume that the disabled node type is always superior and
> > should always be used when it is possible.
>
> In addition to what I said earlier, I think the current
> enable_indexonlyscan is implemented in a way that has the planner do
> what it did before IOS was added.  I think that goal makes sense with
> any patch that make the planner try something new. We want to have
> some method to get the previous behaviour for the cases where the
> planner makes a dumb choice or to avoid some bug in the new feature.

I see the logic of this, and I agree that the resulting behavior might
be more intuitive than what I posted before. I'll do some experiments.

> I think using that logic, the current scenario with enable_indexscan
> and enable_indexonlyscan makes complete sense. I mean, including
> enable_indexscan=0 adding disable_cost to IOS Paths.

This, for me, is a bridge too far. I don't think there's a real
argument that "what the planner did before IOS was added" was add
disable_cost to the cost of index-only scan paths. There was no such
path type. Independently of that argument, I also think the behavior
of a setting needs to be something that a user can understand. Right
now, the documentation says:

Enables or disables the query planner's use of index-scan plan types.
The default is on.
Enables or disables the query planner's use of index-only-scan plan
types (see Section 11.9). The default is on.

I do not think that a user can be expected to guess from these
descriptions that the first one also affects index-only scans, or that
the two GUCs disable their respective plan types in completely
different ways. Granted, the latter inconsistency affects a whole
bunch of these settings, not just this one, but still.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Sat, Nov 2, 2019 at 10:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> The idea that I've been thinking about is to not generate disabled
> Paths in the first place, thus not only fixing the problem but saving
> some cycles.  While this seems easy enough for "optional" paths,
> we have to reserve the ability to generate certain path types regardless,
> if there's no other way to implement the query.  This is a bit of a
> stumbling block :-(.  At the base relation level, we could do something
> like generating seqscan last, and only if no other path has been
> successfully generated.

Continuing my investigation into this rather old thread, I did a
rather primitive implementation of this idea, for baserels only, and
discovered that it caused a small number of planner failures running
the regression tests. Here is a slightly simplified example:

CREATE TABLE strtest (n text, t text);
CREATE INDEX strtest_n_idx ON strtest (n);
SET enable_seqscan=false;
EXPLAIN SELECT * FROM strtest s1 INNER JOIN strtest s2 ON s1.n >= s2.n;

With the patch, I get:

ERROR:  could not devise a query plan for the given query

The problem here is that it's perfectly possible to generate a valid
path for s1 -- and likewise for s2, since it's the same underlying
relation -- while respecting the enable_seqscan=false constraint.
However, all such paths are parameterized by the other of the two
relations, which means that if we do that, we can't plan the join,
because we need an unparameterized path for at least one of the two
sides in order to build a nested loop join, which is the only way to
satisfy the parameterization on the other side.

Now, you could try to fix this by deciding that planning for a baserel
hasn't really succeeded unless we got at least one *unparameterized*
path for that baserel. I haven't tried that, but I presume that if you
do, it fixes the above example, because now there will be a last-ditch
sequential scan on both sides and so this example will behave as
expected. But if you do that, then in other cases, that sequential
scan is going to get picked even when it isn't strictly necessary to
do so, just because some plan that uses it looks better on cost.
Presumably that problem can in turn be fixed by deciding that we also
need to keep disable_cost around (or the separate disable-counter idea
that we were discussing recently in another branch of this thread),
but that's arguably missing the point of this exercise.

Another idea is to remove the ERROR mentioned above from
set_cheapest() and just allow planning to continue even if some
relations end up with no paths. (This would necessitate finding and
fixing any code that could be confused by a pathless relation.) Then,
if you get to the top of the plan tree and you have no paths there,
redo the join search discarding the constraints (or maybe just some of
the constraints, e.g. allow sequential scans and nested loops, or
something). Conceptually, I like this idea a lot, but I think there
are a few problems. One is that I'm not quite sure how to find all the
code that would need to be adjusted to make it work, though the header
comment for standard_join_search() seems like it's got some helpful
tips. A second is that it's another version of the disable_cost =
infinity problem: once you find that you can't generate a path while
enforcing all of the restrictions, you just disregard the restrictions
completely, instead of discarding them only to the extent necessary. I
have a feeling that's not going to be very appealing.

Now, I suppose it might be that even if we can't remove disable_cost,
something along these lines is still worth doing, just to save CPU
cycles. You could for example try planning with only non-disabled
stuff and then do it over again with everything if that doesn't work
out, still keeping disable_cost around so that you avoid disabled
nodes where you can. But I'm kind of hoping that I'm missing something
and there's some approach that could both kill disable_cost and save
some cycles at the same time. If (any of) you have an idea, I'd love
to hear it!

--
Robert Haas
EDB: http://www.enterprisedb.com

On Sat, 4 May 2024 at 08:34, Robert Haas <robertmhaas@gmail.com> wrote:
> Another idea is to remove the ERROR mentioned above from
> set_cheapest() and just allow planning to continue even if some
> relations end up with no paths. (This would necessitate finding and
> fixing any code that could be confused by a pathless relation.) Then,
> if you get to the top of the plan tree and you have no paths there,
> redo the join search discarding the constraints (or maybe just some of
> the constraints, e.g. allow sequential scans and nested loops, or
> something).

I don't think you'd need to wait longer than where we do set_cheapest
and find no paths to find out that there's going to be a problem.

I don't think redoing planning is going to be easy or even useful. I
mean what do you change when you replan? You can't just do
enable_seqscan and enable_nestloop as if there's no index to provide
sorted input and the plan requires some sort, then you still can't
produce a plan.  Adding enable_sort to the list does not give me much
confidence we'll never fail to produce a plan either. It just seems
impossible to know which of the disabled ones caused the RelOptInfo to
have no paths.  Also, you might end up enabling one that caused the
planner to do something different than it would do today.  For
example, a Path that today incurs 2x disable_cost vs a Path that only
receives 1x disable_cost might do something different if you just went
and enabled a bunch of enable* GUCs before replanning.

> Now, I suppose it might be that even if we can't remove disable_cost,
> something along these lines is still worth doing, just to save CPU
> cycles. You could for example try planning with only non-disabled
> stuff and then do it over again with everything if that doesn't work
> out, still keeping disable_cost around so that you avoid disabled
> nodes where you can. But I'm kind of hoping that I'm missing something
> and there's some approach that could both kill disable_cost and save
> some cycles at the same time. If (any of) you have an idea, I'd love
> to hear it!

I think the int Path.disabledness idea is worth coding up to try it.
I imagine that a Path will incur the maximum of its subpath's
disabledness's then add_path() just needs to prefer lower-valued
disabledness Paths.

That doesn't get you the benefits of fewer CPU cycles, but where did
that come from as a motive to change this? There's no shortage of
other ways to make the planner faster if that's an issue.

David

David Rowley <dgrowleyml@gmail.com> writes:
> I don't think you'd need to wait longer than where we do set_cheapest
> and find no paths to find out that there's going to be a problem.

At a base relation, yes, but that doesn't work for joins: it may be
that a particular join cannot be formed, yet other join sequences
will work.  We have that all the time from outer-join ordering
restrictions, never mind enable_xxxjoin flags.  So I'm not sure
that we can usefully declare early failure for joins.

> I think the int Path.disabledness idea is worth coding up to try it.
> I imagine that a Path will incur the maximum of its subpath's
> disabledness's then add_path() just needs to prefer lower-valued
> disabledness Paths.

I would think sum not maximum, but that's a detail.

> That doesn't get you the benefits of fewer CPU cycles, but where did
> that come from as a motive to change this? There's no shortage of
> other ways to make the planner faster if that's an issue.

The concern was to not *add* CPU cycles in order to make this area
better.  But I do tend to agree that we've exhausted all the other
options.

BTW, I looked through costsize.c just now to see exactly what we are
using disable_cost for, and it seemed like a majority of the cases are
just wrong.  Where possible, we should implement a plan-type-disable
flag by not generating the associated Path in the first place, not by
applying disable_cost to it.  But it looks like a lot of people have
erroneously copied the wrong logic.  I would say that only these plan
types should use the disable_cost method:

    seqscan
    nestloop join
    sort

as those are the only ones where we risk not being able to make a
plan at all for lack of other alternatives.

There is also some weirdness around needing to force use of tidscan
if we have WHERE CURRENT OF.  But perhaps a different hack could be
used for that.

We also have this for hashjoin:

     * If the bucket holding the inner MCV would exceed hash_mem, we don't
     * want to hash unless there is really no other alternative, so apply
     * disable_cost.

I'm content to leave that be, if we can't remove disable_cost
entirely.

What I'm wondering at this point is whether we need to trouble with
implementing the separate-disabledness-count method, if we trim back
the number of places using disable_cost to the absolute minimum.

            regards, tom lane

On Sun, 5 May 2024 at 04:57, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> David Rowley <dgrowleyml@gmail.com> writes:
> > That doesn't get you the benefits of fewer CPU cycles, but where did
> > that come from as a motive to change this? There's no shortage of
> > other ways to make the planner faster if that's an issue.
>
> The concern was to not *add* CPU cycles in order to make this area
> better.  But I do tend to agree that we've exhausted all the other
> options.

It really looks to me that Robert was talking about not generating
paths for disabled path types. He did write "just to save CPU cycles"
in the paragraph I quoted.

I think we should concern ourselves with adding overhead to add_path()
*only* when we actually see a patch which slows it down in a way that
we can measure.  I find it hard to imagine that adding a single
comparison for every Path is measurable.  Each of these paths has been
palloced and costed, both of which are significantly more expensive
than adding another comparison to compare_path_costs_fuzzily().  I'm
only willing for benchmarks on an actual patch to prove me wrong on
that. Nothing else. add_path() has become a rat's nest of conditions
over the years and those seem to have made it without concerns about
performance.

> BTW, I looked through costsize.c just now to see exactly what we are
> using disable_cost for, and it seemed like a majority of the cases are
> just wrong.  Where possible, we should implement a plan-type-disable
> flag by not generating the associated Path in the first place, not by
> applying disable_cost to it.  But it looks like a lot of people have
> erroneously copied the wrong logic.  I would say that only these plan
> types should use the disable_cost method:
>
>         seqscan
>         nestloop join
>         sort

I think this oversimplifies the situation.  I only spent 30 seconds
looking and I saw cases where this would cause issues. If
enable_hashagg is false, we could fail to produce some plans where the
type is sortable but not hashable. There's also an issue with nested
loops being unable to FULL OUTER JOIN. However, I do agree that there
are some in there that are adding disable_cost that should be done by
just not creating the Path.  enable_gathermerge is one.
enable_bitmapscan is probably another.

I understand you only talked about the cases adding disable_cost in
costsize.c. But just as a reminder, there are other things we need to
be careful not to break. For example, enable_indexonlyscan=false
should defer to still making an index scan.  Nobody who disables
enable_indexonlyscan without disabling enable_indexscan wants queries
that are eligible to use IOS to use seq scan instead. They'd still
want Index Scan to be considered, otherwise they'd have disabled
enable_indexscan.

David

On Sat, May 4, 2024 at 9:16 AM David Rowley <dgrowleyml@gmail.com> wrote:
> I don't think you'd need to wait longer than where we do set_cheapest
> and find no paths to find out that there's going to be a problem.

I'm confused by this response, because I thought that the main point
of my previous email was explaining why that's not true. I showed an
example where you do find paths at set_cheapest() time and yet are
unable to complete planning.

> I don't think redoing planning is going to be easy or even useful. I
> mean what do you change when you replan? You can't just do
> enable_seqscan and enable_nestloop as if there's no index to provide
> sorted input and the plan requires some sort, then you still can't
> produce a plan.  Adding enable_sort to the list does not give me much
> confidence we'll never fail to produce a plan either. It just seems
> impossible to know which of the disabled ones caused the RelOptInfo to
> have no paths.  Also, you might end up enabling one that caused the
> planner to do something different than it would do today.  For
> example, a Path that today incurs 2x disable_cost vs a Path that only
> receives 1x disable_cost might do something different if you just went
> and enabled a bunch of enable* GUCs before replanning.

I agree that there are problems here, both in terms of implementation
complexity and also in terms of what behavior you actually get, but I
do not think that a proposal which changes some current behavior
should be considered dead on arrival. Whatever new behavior we might
want to implement needs to make sense, and there need to be good
reasons for making whatever changes are contemplated, but I don't
think we should take the position that it has to be identical to
current.

> I think the int Path.disabledness idea is worth coding up to try it.
> I imagine that a Path will incur the maximum of its subpath's
> disabledness's then add_path() just needs to prefer lower-valued
> disabledness Paths.

It definitely needs to be sum, not max. Otherwise you can't get the
matest example from the regression tests right, where one child lacks
the ability to comply with the GUC setting.

> That doesn't get you the benefits of fewer CPU cycles, but where did
> that come from as a motive to change this? There's no shortage of
> other ways to make the planner faster if that's an issue.

Well, I don't agree with that at all. If there are lots of ways to
make the planner faster, we should definitely do a bunch of that
stuff, because "will slow down the planner too much" has been a
leading cause of proposed planner patches being rejected for as long
as I've been involved with the project. My belief was that we were
rather short of good ideas in that area, actually. But even if it's
true that we have lots of other ways to speed up the planner, that
doesn't mean that it wouldn't be good to do it here, too.

Stepping back a bit, my current view of this area is: disable_cost is
highly imperfect both as an idea and as implemented in PostgreSQL.
Although I'm discovering that the current implementation gets more
things right than I had realized, it also sometimes gets things wrong.
The original poster gave an example of that, and there are others.
Furthermore, the current implementation has some weird
inconsistencies. Therefore, I would like something better. Better, to
me, could mean any combination of (a) superior behavior, (b) superior
performance, and (c) simpler, more elegant code. In a perfect world,
we'd be able to come up with something that wins in all three of those
areas, but I'm not seeing a way to achieve that, so I'm trying to
figure out what is achievable. And because we need to reach consensus
on whatever is to be done, I'm sharing raw research results rather
than just dropping a completed patch. I don't think it's at all easy
to understand what the realistic possibilities are in this area;
certainly it isn't for me. At some point I'm hoping that there will be
a patch (or a bunch of patches) that we can all agree are an
improvement over now and the best we can reasonably do, but I don't
yet know what the shape of those will be, because I'm still trying to
understand (and document on-list) what all the problems are.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Sat, May 4, 2024 at 12:57 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> There is also some weirdness around needing to force use of tidscan
> if we have WHERE CURRENT OF.  But perhaps a different hack could be
> used for that.

Yeah, figuring out what to do about this was the trickiest part of the
experimental patch that I wrote last week. The idea of the current
code is that cost_qual_eval_walker charges disable_cost for
CurrentOfExpr, but cost_tidscan then subtracts disable_cost if
tidquals contains a CurrentOfExpr, so that we effectively disable
everything except TID scan paths and, I think, also any TID scan paths
that don't use the CurrentOfExpr as a qual. I'm not entirely sure
whether the last can happen, but I imagine that it might be possible
if the cursor refers to a query that itself contains some other kind
of TID qual.

It's not very clear that this mechanism is actually 100% reliable,
because we know it's possible in general for the costs of two paths to
be different by more than disable_cost. Maybe that's not possible in
this specific context, though: I'm not sure.

The approach I took for my experimental patch was pretty grotty, and
probably not quite complete, but basically I defined the case where we
currently subtract out disable_cost as a "forced TID-scan". I passed
around a Boolean called forcedTidScan which gets set to true if we
discover that some plan is a forced TID-scan path, and then we discard
any other paths and then only add other forced TID-scan paths after
that point. There can be more than one, because of parameterization.

But I think that the right thing to do is probably to pull some of the
logic up out of create_tidscan_paths() and decide ONCE whether we're
in a forced TID-scan situation or not. If we are, then
set_plain_rel_pathlist() should arrange to create only forced TID-scan
paths; otherwise, it should proceed as it does now.

Maybe if I try to do that I'll find problems, but the current approach
seems backwards to me, like going to a restaurant and ordering one of
everything on the menu, then cancelling all of the orders except the
stuff you actually want.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Mon, May 6, 2024 at 9:39 AM Robert Haas <robertmhaas@gmail.com> wrote:
> It's not very clear that this mechanism is actually 100% reliable,

It isn't. Here's a test case. As a non-superuser, do this:

create table foo (a int, b text, primary key (a));
insert into foo values (1, 'Apple');
alter table foo enable row level security;
alter table foo force row level security;
create policy p1 on foo as permissive using (ctid in ('(0,1)', '(0,2)'));
begin;
declare c cursor for select * from foo;
fetch from c;
explain update foo set b = 'Manzana' where current of c;
update foo set b = 'Manzana' where current of c;

The explain produces this output:

 Update on foo  (cost=10000000000.00..10000000008.02 rows=0 width=0)
   ->  Tid Scan on foo  (cost=10000000000.00..10000000008.02 rows=1 width=38)
         TID Cond: (ctid = ANY ('{"(0,1)","(0,2)"}'::tid[]))
         Filter: CURRENT OF c

Unless I'm quite confused, the point of the code is to force
CurrentOfExpr to be a TID Cond, and it normally succeeds in doing so,
because WHERE CURRENT OF cursor_name has to be the one and only WHERE
condition for a normal UPDATE. I tried various cases involving views
and CTEs and got nowhere. But then I wrote a patch to make the
regression tests fail if a baserel's restrictinfo list contains a
CurrentOfExpr and also some other qual, and a couple of row-level
security tests failed (and nothing else). Which then allowed me to
construct the example above, where there are two possible TID quals
and the logic in tidpath.c latches onto the wrong one. The actual
UPDATE fails like this:

ERROR:  WHERE CURRENT OF is not supported for this table type

...because ExecEvalCurrentOfExpr supposes that the only way it can be
reached is for an FDW without the necessary support, but actually in
this case it's planner error that gets us here.

Fortunately, there's no real reason for anyone to ever do something
like this, or at least I can't see one, so the fact that it doesn't
work probably doesn't really matter that much. And you can argue that
the only problem here is that the costing hack just didn't get updated
for RLS and now needs to be a bit more clever. But I think it'd be
better to find a way of making it less hacky. With the way the code is
structured right now, the chances of anyone understanding that RLS
might have an impact on its correctness were just about nil, IMHO.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, May 6, 2024 at 9:39 AM Robert Haas <robertmhaas@gmail.com> wrote:
>> It's not very clear that this mechanism is actually 100% reliable,

> It isn't. Here's a test case.

Very interesting.

> ... Which then allowed me to
> construct the example above, where there are two possible TID quals
> and the logic in tidpath.c latches onto the wrong one.

Hmm.  Without having traced through it, I'm betting that the
CurrentOfExpr qual is rejected as a tidqual because it's not
considered leakproof.  It's not obvious to me why we couldn't consider
it as leakproof, though.  If we don't want to do that in general,
then we need some kind of hack in TidQualFromRestrictInfo to accept
CurrentOfExpr quals anyway.

In general I think you're right that something less rickety than
the disable_cost hack would be a good idea to ensure the desired
TidPath gets chosen, but this problem is not the fault of that.
We're not making the TidPath with the correct contents in the first
place.

            regards, tom lane

I wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> ... Which then allowed me to
>> construct the example above, where there are two possible TID quals
>> and the logic in tidpath.c latches onto the wrong one.

> Hmm.  Without having traced through it, I'm betting that the
> CurrentOfExpr qual is rejected as a tidqual because it's not
> considered leakproof.

Nah, I'm wrong: we do treat it as leakproof, and the comment about
that in contain_leaked_vars_walker shows that the interaction with
RLS quals *was* thought about.  What wasn't thought about was the
possibility of RLS quals that themselves could be usable as tidquals,
which breaks this assumption in TidQualFromRestrictInfoList:

         * Stop as soon as we find any usable CTID condition.  In theory we
         * could get CTID equality conditions from different AND'ed clauses,
         * in which case we could try to pick the most efficient one.  In
         * practice, such usage seems very unlikely, so we don't bother; we
         * just exit as soon as we find the first candidate.

The executor doesn't seem to be prepared to cope with multiple AND'ed
TID clauses (only OR'ed ones).  So we need to fix this at least to the
extent of looking for a CurrentOfExpr qual, and preferring that over
anything else.

I'm also now wondering about this assumption in the executor:

    /* CurrentOfExpr could never appear OR'd with something else */
    Assert(list_length(tidstate->tss_tidexprs) == 1 ||
           !tidstate->tss_isCurrentOf);

It still seems OK, because anything that might come in from RLS quals
would be AND'ed not OR'ed with the CurrentOfExpr.

> In general I think you're right that something less rickety than
> the disable_cost hack would be a good idea to ensure the desired
> TidPath gets chosen, but this problem is not the fault of that.
> We're not making the TidPath with the correct contents in the first
> place.

Still true.

            regards, tom lane

On Mon, May 6, 2024 at 3:26 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Nah, I'm wrong: we do treat it as leakproof, and the comment about
> that in contain_leaked_vars_walker shows that the interaction with
> RLS quals *was* thought about.  What wasn't thought about was the
> possibility of RLS quals that themselves could be usable as tidquals,
> which breaks this assumption in TidQualFromRestrictInfoList:
>
>          * Stop as soon as we find any usable CTID condition.  In theory we
>          * could get CTID equality conditions from different AND'ed clauses,
>          * in which case we could try to pick the most efficient one.  In
>          * practice, such usage seems very unlikely, so we don't bother; we
>          * just exit as soon as we find the first candidate.

Right. I had noticed this but didn't spell it out.

> The executor doesn't seem to be prepared to cope with multiple AND'ed
> TID clauses (only OR'ed ones).  So we need to fix this at least to the
> extent of looking for a CurrentOfExpr qual, and preferring that over
> anything else.
>
> I'm also now wondering about this assumption in the executor:
>
>     /* CurrentOfExpr could never appear OR'd with something else */
>     Assert(list_length(tidstate->tss_tidexprs) == 1 ||
>            !tidstate->tss_isCurrentOf);
>
> It still seems OK, because anything that might come in from RLS quals
> would be AND'ed not OR'ed with the CurrentOfExpr.

This stuff I had not noticed.

> > In general I think you're right that something less rickety than
> > the disable_cost hack would be a good idea to ensure the desired
> > TidPath gets chosen, but this problem is not the fault of that.
> > We're not making the TidPath with the correct contents in the first
> > place.
>
> Still true.

I'll look into this, unless you want to do it.

Incidentally, another thing I just noticed is that
IsCurrentOfClause()'s test for (node->cvarno == rel->relid) is
possibly dead code. At least, there are no examples in our test suite
where it fails to hold. Which seems like it makes sense, because if it
didn't, then how did the clause end up in baserestrictinfo? Maybe this
is worth keeping as defensive coding, or maybe it should be changed to
an Assert or something.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> I'll look into this, unless you want to do it.

I have a draft patch already.  Need to add a test case.

> Incidentally, another thing I just noticed is that
> IsCurrentOfClause()'s test for (node->cvarno == rel->relid) is
> possibly dead code. At least, there are no examples in our test suite
> where it fails to hold. Which seems like it makes sense, because if it
> didn't, then how did the clause end up in baserestrictinfo? Maybe this
> is worth keeping as defensive coding, or maybe it should be changed to
> an Assert or something.

I wouldn't remove it, but maybe an Assert is good enough.  The tests
on Vars' varno should be equally pointless no?

            regards, tom lane

On Mon, May 6, 2024 at 8:27 AM Robert Haas <robertmhaas@gmail.com> wrote:
> Stepping back a bit, my current view of this area is: disable_cost is
> highly imperfect both as an idea and as implemented in PostgreSQL.
> Although I'm discovering that the current implementation gets more
> things right than I had realized, it also sometimes gets things wrong.
> The original poster gave an example of that, and there are others.
> Furthermore, the current implementation has some weird
> inconsistencies. Therefore, I would like something better.

FWIW I always found those weird inconsistencies to be annoying at
best, and confusing at worst. I speak as somebody that uses
disable_cost a lot.

I certainly wouldn't ask anybody to make it a priority for that reason
alone -- it's not *that* bad. I've given my opinion on this because
it's already under discussion.

--
Peter Geoghegan

On Mon, May 6, 2024 at 4:30 PM Peter Geoghegan <pg@bowt.ie> wrote:
> FWIW I always found those weird inconsistencies to be annoying at
> best, and confusing at worst. I speak as somebody that uses
> disable_cost a lot.
>
> I certainly wouldn't ask anybody to make it a priority for that reason
> alone -- it's not *that* bad. I've given my opinion on this because
> it's already under discussion.

Thanks, it's good to have other perspectives.

Here are some patches for discussion.

0001 gets rid of disable_cost as a mechanism for forcing a TID scan
plan to be chosen when CurrentOfExpr is present. Instead, it arranges
to generate only the valid path when that case occurs, and skip
everything else. I think this is a good cleanup, and it doesn't seem
totally impossible that it actually prevents a failure in some extreme
case.

0002 cleans up the behavior of enable_indexscan and
enable_indexonlyscan. Currently, setting enable_indexscan=false adds
disable_cost to both the cost of index scans and the cost of
index-only scans. I think that's indefensible and, in fact, a bug,
although I believe David Rowley disagrees. With this patch, we simply
don't generate index scans if enable_indexscan=false, and we don't
generate index-only scans if enable_indexonlyscan=false, which seems a
lot more consistent to me. However, I did revise one major thing from
the patch I posted before, per feedback from David Rowley and also per
my own observations: in this version, if enable_indexscan=true and
enable_indexonlyscan=false, we'll generate index-scan paths for any
cases where, with both set to true, we would have only generated
index-only scan paths. That change makes the behavior of this patch a
lot more comprehensible and intuitive: the only regression test
changes are places where somebody expected that they could disable
both index scans and index-only scans by setting
enable_indexscan=false.

0003 and 0004 extend the approach of "just don't generate the disabled
path" to bitmap scans and gather merge, respectively. I think these
are more debatable, mostly because it's not clear how far we can
really take this approach. Neither breaks any test cases, and 0003 is
closely related to the work done in 0002, which seems like a point in
its favor. 0004 was simply the only other case where it was obvious to
me that this kind of approach made sense. In my view, it makes most
sense to use this kind of approach for planner behaviors that seem
like they're sort of optional: like if you don't use gather merge, you
can still use gather, and if you don't use index scans, you can still
use sequential scans. With all these patches applied, the remaining
cases where we rely on disable_cost are:

sequential scans
sorts
hash aggregation
all 3 join types
hash joins where a bucket holding the inner MCV would exceed hash_mem

Sequential scans are clearly a last-ditch method. I find it a bit hard
to decide whether hashing or sorting is the default, especially giving
the asymmetry between enable_sort - presumptively anywhere - and
enable_hashagg - specific to aggregation. As for the join types, it's
tempting to consider nested-loop the default type -- it's the only way
to satisfy parameterizations, for instance -- but the fact that it's
the only method that can't do a full join undermines that position in
my book. But, I don't want to pretend like I have all the answers
here, either; I'm just sharing some thoughts.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Tue, May 7, 2024 at 4:19 PM Robert Haas <robertmhaas@gmail.com> wrote:
> Here are some patches for discussion.

Well, that didn't generate much discussion, but here I am trying
again. Here I've got patches 0001 and 0002 from my previous posting;
I've dropped 0003 and 0004 from the previous set for now so as not to
distract from the main event, but they may still be a good idea.
Instead I've got an 0003 and an 0004 that implement the "count of
disabled nodes" approach that we have discussed previously. This seems
to work fine, unlike the approaches I tried earlier. I think this is
the right direction to go, but I'd like to know what concerns people
might have.

This doesn't completely remove disable_cost, because hash joins still
add it to the cost when it's impossible to fit the MCV value into
work_mem. I'm not sure what to do with that. Continuing to use
disable_cost in that one scenario seems OK to me. We could
alternatively make that scenario bump disabled_nodes, but I don't
really want to confuse the planner not wanting to do something with
the user telling the planner not to do something, so I don't think
that's a good idea. Or we could rejigger things so that in that case
we don't generate the plan at all. I'm not sure why we don't do that
already, actually.

--
Robert Haas
EDB: http://www.enterprisedb.com

Hi,

On 2024-06-12 11:35:48 -0400, Robert Haas wrote:
> Subject: [PATCH v2 3/4] Treat the # of disabled nodes in a path as a separate
>  cost metric.
> 
> Previously, when a path type was disabled by e.g. enable_seqscan=false,
> we either avoided generating that path type in the first place, or
> more commonly, we added a large constant, called disable_cost, to the
> estimated startup cost of that path. This latter approach can distort
> planning. For instance, an extremely expensive non-disabled path
> could seem to be worse than a disabled path, especially if the full
> cost of that path node need not be paid (e.g. due to a Limit).
> Or, as in the regression test whose expected output changes with this
> commit, the addition of disable_cost can make two paths that would
> normally be distinguishible cost seem to have fuzzily the same cost.
> 
> To fix that, we now count the number of disabled path nodes and
> consider that a high-order component of both the cost. Hence, the
> path list is now sorted by disabled_nodes and then by total_cost,
> instead of just by the latter, and likewise for the partial path list.
> It is important that this number is a count and not simply a Boolean;
> else, as soon as we're unable to respect disabled path types in all
> portions of the path, we stop trying to avoid them where we can.


>      if (criterion == STARTUP_COST)
>      {
>          if (path1->startup_cost < path2->startup_cost)
> @@ -118,6 +127,15 @@ compare_fractional_path_costs(Path *path1, Path *path2,
>      Cost        cost1,
>                  cost2;
>  
> +    /* Number of disabled nodes, if different, trumps all else. */
> +    if (unlikely(path1->disabled_nodes != path2->disabled_nodes))
> +    {
> +        if (path1->disabled_nodes < path2->disabled_nodes)
> +            return -1;
> +        else
> +            return +1;
> +    }

I suspect it's going to be ok, because the branch is going to be very
predictable in normal workloads, but I still worry a bit about making
compare_path_costs_fuzzily() more expensive. For more join-heavy queries it
can really show up and there's plenty ORM generated join-heavy query
workloads.

If costs were 32 bit integers, I'd have suggested just stashing the disabled
counts in the upper 32 bits of a 64bit integer. But ...

<can't resist trying if I see overhead>


In an extreme case i can see a tiny bit of overhead, but not enough to be
worth worrying about. Mostly because we're so profligate in doing
bms_overlap() that cost comparisons don't end up mattering as much - I seem to
recall that being different in the not distant past though.


Aside: I'm somewhat confused by add_paths_to_joinrel()'s handling of
mergejoins_allowed. If mergejoins are disabled we end up reaching
match_unsorted_outer() in more cases than with mergejoins enabled. E.g. we
only set mergejoin_enabled for right joins inside select_mergejoin_clauses(),
but we don't call select_mergejoin_clauses() if !enable_mergejoin and jointype
!= FULL.  I, what?


Greetings,

Andres Freund

On Wed, Jun 12, 2024 at 2:11 PM Andres Freund <andres@anarazel.de> wrote:
> <can't resist trying if I see overhead>
>
> In an extreme case i can see a tiny bit of overhead, but not enough to be
> worth worrying about. Mostly because we're so profligate in doing
> bms_overlap() that cost comparisons don't end up mattering as much - I seem to
> recall that being different in the not distant past though.

There are very few things I love more than when you can't resist
trying to break my patches and yet fail to find a problem. Granted the
latter part only happens once a century or so, but I'll take it.

> Aside: I'm somewhat confused by add_paths_to_joinrel()'s handling of
> mergejoins_allowed. If mergejoins are disabled we end up reaching
> match_unsorted_outer() in more cases than with mergejoins enabled. E.g. we
> only set mergejoin_enabled for right joins inside select_mergejoin_clauses(),
> but we don't call select_mergejoin_clauses() if !enable_mergejoin and jointype
> != FULL.  I, what?

I agree this logic is extremely confusing, but "we only set
mergejoin_enabled for right joins inside select_mergejoin_clauses()"
doesn't seem to be true. It starts out true, and always stays true
except for right, right-anti, and full joins, where
select_mergejoin_clauses() can set it to false. Since the call to
match_unsorted_outer() is gated by mergejoin_enabled, you might think
that we'd skip considering nested loops on the strength of not being
able to do a merge join, but comment "2." in add_paths_to_joinrel
explains that the join types for which mergejoin_enabled can end up
false aren't supported by nested loops anyway. Still, this logic is
really tortured.

--
Robert Haas
EDB: http://www.enterprisedb.com

Hi,

On 2024-06-12 14:33:31 -0400, Robert Haas wrote:
> On Wed, Jun 12, 2024 at 2:11 PM Andres Freund <andres@anarazel.de> wrote:
> > <can't resist trying if I see overhead>
> >
> > In an extreme case i can see a tiny bit of overhead, but not enough to be
> > worth worrying about. Mostly because we're so profligate in doing
> > bms_overlap() that cost comparisons don't end up mattering as much - I seem to
> > recall that being different in the not distant past though.
> 
> There are very few things I love more than when you can't resist
> trying to break my patches and yet fail to find a problem. Granted the
> latter part only happens once a century or so, but I'll take it.

:)


Too high cost in path cost comparison is what made me look at the PG code for
the first time, IIRC :)



> > Aside: I'm somewhat confused by add_paths_to_joinrel()'s handling of
> > mergejoins_allowed. If mergejoins are disabled we end up reaching
> > match_unsorted_outer() in more cases than with mergejoins enabled. E.g. we
> > only set mergejoin_enabled for right joins inside select_mergejoin_clauses(),
> > but we don't call select_mergejoin_clauses() if !enable_mergejoin and jointype
> > != FULL.  I, what?
> 
> I agree this logic is extremely confusing, but "we only set
> mergejoin_enabled for right joins inside select_mergejoin_clauses()"
> doesn't seem to be true.

Sorry, should have been more precise. With "set" I didn't mean set to true,
but that that it's only modified within select_mergejoin_clauses().


> It starts out true, and always stays true except for right, right-anti, and
> full joins, where select_mergejoin_clauses() can set it to false. Since the
> call to match_unsorted_outer() is gated by mergejoin_enabled, you might
> think that we'd skip considering nested loops on the strength of not being
> able to do a merge join, but comment "2." in add_paths_to_joinrel explains
> that the join types for which mergejoin_enabled can end up false aren't
> supported by nested loops anyway. Still, this logic is really tortured.

Agree that that's the logic - but doesn't that mean we'll consider nestloops
for e.g. right joins iff enable_mergejoin=false?


Greetings,

Andres Freund

On Wed, Jun 12, 2024 at 2:48 PM Andres Freund <andres@anarazel.de> wrote:
> Sorry, should have been more precise. With "set" I didn't mean set to true,
> but that that it's only modified within select_mergejoin_clauses().

Oh. "set" has more than one relevant meaning here.

> > It starts out true, and always stays true except for right, right-anti, and
> > full joins, where select_mergejoin_clauses() can set it to false. Since the
> > call to match_unsorted_outer() is gated by mergejoin_enabled, you might
> > think that we'd skip considering nested loops on the strength of not being
> > able to do a merge join, but comment "2." in add_paths_to_joinrel explains
> > that the join types for which mergejoin_enabled can end up false aren't
> > supported by nested loops anyway. Still, this logic is really tortured.
>
> Agree that that's the logic - but doesn't that mean we'll consider nestloops
> for e.g. right joins iff enable_mergejoin=false?

No, because that function has its own internal guards. See nestjoinOK.

But don't misunderstand me: I'm not defending the status quo. The
whole thing seems like a Rube Goldberg machine to me.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Wed, Jun 12, 2024 at 11:35 AM Robert Haas <robertmhaas@gmail.com> wrote:
> Well, that didn't generate much discussion, but here I am trying
> again. Here I've got patches 0001 and 0002 from my previous posting;
> I've dropped 0003 and 0004 from the previous set for now so as not to
> distract from the main event, but they may still be a good idea.
> Instead I've got an 0003 and an 0004 that implement the "count of
> disabled nodes" approach that we have discussed previously. This seems
> to work fine, unlike the approaches I tried earlier. I think this is
> the right direction to go, but I'd like to know what concerns people
> might have.

Here is a rebased patch set, where I also fixed pgindent damage and a
couple of small oversights in 0004.

I am hoping to get these committed some time in July. So if somebody
thinks that's too soon or thinks it shouldn't happen at all, please
don't wait too long to let me know about that.

Thanks,

--
Robert Haas
EDB: http://www.enterprisedb.com

On 28/06/2024 18:46, Robert Haas wrote:
> On Wed, Jun 12, 2024 at 11:35 AM Robert Haas <robertmhaas@gmail.com> wrote:
>> Well, that didn't generate much discussion, but here I am trying
>> again. Here I've got patches 0001 and 0002 from my previous posting;
>> I've dropped 0003 and 0004 from the previous set for now so as not to
>> distract from the main event, but they may still be a good idea.
>> Instead I've got an 0003 and an 0004 that implement the "count of
>> disabled nodes" approach that we have discussed previously. This seems
>> to work fine, unlike the approaches I tried earlier. I think this is
>> the right direction to go, but I'd like to know what concerns people
>> might have.
> 
> Here is a rebased patch set, where I also fixed pgindent damage and a
> couple of small oversights in 0004.
> 
> I am hoping to get these committed some time in July. So if somebody
> thinks that's too soon or thinks it shouldn't happen at all, please
> don't wait too long to let me know about that.

v3-0001-Remove-grotty-use-of-disable_cost-for-TID-scan-pl.patch:

+1, this seems ready for commit

v3-0002-Rationalize-behavior-of-enable_indexscan-and-enab.patch:

I fear this will break people's applications, if they are currently 
forcing a sequential scan with "set enable_indexscan=off". Now they will 
need to do "set enable_indexscan=off; set enable_indexonlyscan=off" for 
the same effect. Maybe it's acceptable, disabling sequential scans to 
force an index scan is much more common than the other way round.

v3-0003-Treat-number-of-disabled-nodes-in-a-path-as-a-sep.patch:

> @@ -1318,6 +1342,12 @@ cost_tidscan(Path *path, PlannerInfo *root,
>      startup_cost += path->pathtarget->cost.startup;
>      run_cost += path->pathtarget->cost.per_tuple * path->rows;
>  
> +    /*
> +     * There are assertions above verifying that we only reach this function
> +     * either when enable_tidscan=true or when the TID scan is the only legal
> +     * path, so it's safe to set disabled_nodes to zero here.
> +     */
> +    path->disabled_nodes = 0;
>      path->startup_cost = startup_cost;
>      path->total_cost = startup_cost + run_cost;
>  }

So if you have enable_tidscan=off, and have a query with "WHERE CURRENT 
OF foo" that is planned with a TID scan, we set disable_nodes = 0? That 
sounds wrong, shouldn't disable_nodes be 1 in that case? It probably 
cannot affect the rest of the plan, given that "WHERE CURRENT OF" is 
only valid in an UPDATE or DELETE, but still. At least it deserves a 
better explanation in the comment.

> diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
> index 6b64c4a362..20236e8c4d 100644
> --- a/src/backend/optimizer/plan/createplan.c
> +++ b/src/backend/optimizer/plan/createplan.c
> @@ -25,6 +25,7 @@
>  #include "nodes/extensible.h"
>  #include "nodes/makefuncs.h"
>  #include "nodes/nodeFuncs.h"
> +#include "nodes/print.h"
>  #include "optimizer/clauses.h"
>  #include "optimizer/cost.h"
>  #include "optimizer/optimizer.h"

left over from debugging?

> @@ -68,6 +68,15 @@ static bool pathlist_is_reparameterizable_by_child(List *pathlist,
>  int
>  compare_path_costs(Path *path1, Path *path2, CostSelector criterion)
>  {
> +    /* Number of disabled nodes, if different, trumps all else. */
> +    if (unlikely(path1->disabled_nodes != path2->disabled_nodes))
> +    {
> +        if (path1->disabled_nodes < path2->disabled_nodes)
> +            return -1;
> +        else
> +            return +1;
> +    }
> +
>      if (criterion == STARTUP_COST)
>      {
>          if (path1->startup_cost < path2->startup_cost)

Is "unlikely()" really appropriate here (and elsewhere in the patch)? If 
you run with enable_seqscan=off but have no indexes, you could take that 
path pretty often.

If this function needs optimizing, I'd suggest splitting it into two 
functions, one for comparing the startup cost and another for the total 
cost. Almost all callers pass a constant for that argument, so they 
might as well call the correct function directly and avoid the branch 
for that.

> @@ -658,6 +704,20 @@ add_path_precheck(RelOptInfo *parent_rel,
>          Path       *old_path = (Path *) lfirst(p1);
>          PathKeysComparison keyscmp;
>  
> +        /*
> +         * Since the pathlist is sorted by disabled_nodes and then by
> +         * total_cost, we can stop looking once we reach a path with more
> +         * disabled nodes, or the same number of disabled nodes plus a
> +         * total_cost larger than the new path's.
> +         */
> +        if (unlikely(old_path->disabled_nodes != disabled_nodes))
> +        {
> +            if (disabled_nodes < old_path->disabled_nodes)
> +                break;
> +        }
> +        else if (total_cost <= old_path->total_cost * STD_FUZZ_FACTOR)
> +            break;
> +
>          /*
>           * We are looking for an old_path with the same parameterization (and
>           * by assumption the same rowcount) that dominates the new path on
> @@ -666,39 +726,27 @@ add_path_precheck(RelOptInfo *parent_rel,
>           *
>           * Cost comparisons here should match compare_path_costs_fuzzily.
>           */
> -        if (total_cost > old_path->total_cost * STD_FUZZ_FACTOR)
> +        /* new path can win on startup cost only if consider_startup */
> +        if (startup_cost > old_path->startup_cost * STD_FUZZ_FACTOR ||
> +            !consider_startup)
>          {

The "Cost comparisons here should match compare_path_costs_fuzzily" 
comment also applies to the check on total_cost that you moved up. Maybe 
move up the comment to the beginning of the loop.

v3-0004-Show-number-of-disabled-nodes-in-EXPLAIN-ANALYZE-.patch:

It's surprising that the "Disable Nodes" is printed even with the COSTS 
OFF option. It's handy for our regression tests, it's good to print them 
there, but it feels wrong.

Could we cram it into the "cost=... rows=..." part? And perhaps a marker 
that a node was disabled would be more user friendly than showing the 
cumulative count? Something like:

postgres=# set enable_material=off;
SET
postgres=# set enable_seqscan=off;
SET
postgres=# set enable_bitmapscan=off;
SET
postgres=# explain select * from foo, bar;
                                      QUERY PLAN 

------------------------------------------------------------------------------------
  Nested Loop  (cost=0.15..155632.40 rows=6502500 width=8)
    ->  Index Only Scan using foo_i_idx on foo  (cost=0.15..82.41 
rows=2550 width=4)
    ->  Seq Scan on bar  (cost=0.00..35.50 (disabled) rows=2550 width=4)
(5 rows)

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Thanks for the review!

On Tue, Jul 2, 2024 at 10:57 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> v3-0001-Remove-grotty-use-of-disable_cost-for-TID-scan-pl.patch:
>
> +1, this seems ready for commit

Cool.

> v3-0002-Rationalize-behavior-of-enable_indexscan-and-enab.patch:
>
> I fear this will break people's applications, if they are currently
> forcing a sequential scan with "set enable_indexscan=off". Now they will
> need to do "set enable_indexscan=off; set enable_indexonlyscan=off" for
> the same effect. Maybe it's acceptable, disabling sequential scans to
> force an index scan is much more common than the other way round.

Well, I think it's pretty important that the GUC does what the name
and documentation say it does. One could of course argue that we ought
not to have two different GUCs -- or perhaps even that we ought not to
have two different plan nodes -- and I think those arguments might be
quite defensible. One could also argue for another interface, like a
GUC enable_indexscan and a value that is a comma-separated list
consisting of plain, bitmap, and index-only, or
none/0/false/any/1/true -- and that might also be quite defensible.
But I don't think one can have a GUC called enable_indexscan and
another GUC called enable_indexonlyscan and argue that it's OK for the
former one to affect both kinds of scans. That's extremely confusing
and, well, just plain wrong. I think this is a bug, and I'm not going
to back-patch the fix precisely because of the considerations you
note, but I really don't think we can leave it like this. The current
behavior is so nonsensical that the code is essentially unmaintable,
or at least I think it is.

> v3-0003-Treat-number-of-disabled-nodes-in-a-path-as-a-sep.patch:
>
> > @@ -1318,6 +1342,12 @@ cost_tidscan(Path *path, PlannerInfo *root,
> >       startup_cost += path->pathtarget->cost.startup;
> >       run_cost += path->pathtarget->cost.per_tuple * path->rows;
> >
> > +     /*
> > +      * There are assertions above verifying that we only reach this function
> > +      * either when enable_tidscan=true or when the TID scan is the only legal
> > +      * path, so it's safe to set disabled_nodes to zero here.
> > +      */
> > +     path->disabled_nodes = 0;
> >       path->startup_cost = startup_cost;
> >       path->total_cost = startup_cost + run_cost;
> >  }
>
> So if you have enable_tidscan=off, and have a query with "WHERE CURRENT
> OF foo" that is planned with a TID scan, we set disable_nodes = 0? That
> sounds wrong, shouldn't disable_nodes be 1 in that case? It probably
> cannot affect the rest of the plan, given that "WHERE CURRENT OF" is
> only valid in an UPDATE or DELETE, but still. At least it deserves a
> better explanation in the comment.

So, right now, when the planner disregards enable_WHATEVER because it
thinks it's the only way to implement something, it doesn't add
disable_cost. So, I made the patch not increment disabled_nodes in
that case. Maybe we want to rethink that choice at some point, but it
doesn't seem like a good idea to do it right now. I've found while
working on this stuff that it's super-easy to have seemingly innocuous
changes disturb regression test results, and I don't really want to
have a bunch of extra regression test changes that are due to
rethinking things other than disable_cost -> disabled_nodes. So for
now I'd like to increment disabled_nodes in just the cases where we
currently add disable_cost.

> left over from debugging?

Yeah, will fix.

> Is "unlikely()" really appropriate here (and elsewhere in the patch)? If
> you run with enable_seqscan=off but have no indexes, you could take that
> path pretty often.

That's true, but I think it's right to assume that's the uncommon
case. If we speed up planning for people who disabled sequential scans
and slow it down for people running with a normal planner
configuration, no one will thank us.

> If this function needs optimizing, I'd suggest splitting it into two
> functions, one for comparing the startup cost and another for the total
> cost. Almost all callers pass a constant for that argument, so they
> might as well call the correct function directly and avoid the branch
> for that.

That's not a bad idea but seems like a separate patch.

> The "Cost comparisons here should match compare_path_costs_fuzzily"
> comment also applies to the check on total_cost that you moved up. Maybe
> move up the comment to the beginning of the loop.

Will have a look.

> v3-0004-Show-number-of-disabled-nodes-in-EXPLAIN-ANALYZE-.patch:
>
> It's surprising that the "Disable Nodes" is printed even with the COSTS
> OFF option. It's handy for our regression tests, it's good to print them
> there, but it feels wrong.

I'm open to doing what people think is best here. Although we're
regarding them as part of the cost for purposes of how to compare
paths, they're not unpredictable in the way that costs are, so I think
the current handling is defensible and, as you say, it's useful for
the regression tests. However, I'm not going to fight tooth and nail
if people really want it the other way.

> Could we cram it into the "cost=... rows=..." part? And perhaps a marker
> that a node was disabled would be more user friendly than showing the
> cumulative count? Something like:

The problem is that we'd have to derive that. What we actually know is
the disable count; to figure out whether the node itself was disabled,
we'd have to subtract the value for the underlying nodes back out.
That seems like it might be buggy or confusing.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Jul 2, 2024 at 10:57 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> I fear this will break people's applications, if they are currently
>> forcing a sequential scan with "set enable_indexscan=off". Now they will
>> need to do "set enable_indexscan=off; set enable_indexonlyscan=off" for
>> the same effect. Maybe it's acceptable, disabling sequential scans to
>> force an index scan is much more common than the other way round.

> But I don't think one can have a GUC called enable_indexscan and
> another GUC called enable_indexonlyscan and argue that it's OK for the
> former one to affect both kinds of scans. That's extremely confusing
> and, well, just plain wrong.

FWIW, I disagree completely.  I think it's entirely natural to
consider bitmap index scans to be a subset of index scans, so that
enable_indexscan should affect both.  I admit that the current set
of GUCs doesn't let you force a bitmap scan over a plain one, but
I can't recall many people complaining about that.  I don't follow
the argument that this definition is somehow unmaintainable, either.

>> Could we cram it into the "cost=... rows=..." part? And perhaps a marker
>> that a node was disabled would be more user friendly than showing the
>> cumulative count? Something like:

> The problem is that we'd have to derive that.

The other problem is it'd break an awful lot of client code that knows
the format of those lines.  (Sure, by now all such code should have
been rewritten to look at JSON or other more machine-friendly output
formats ... but since we haven't even done that in our own regression
tests, we should know better than to assume other people have done it.)

I'm not really convinced that we need to show anything about this.

            regards, tom lane

On Tue, Jul 2, 2024 at 1:40 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> FWIW, I disagree completely.  I think it's entirely natural to
> consider bitmap index scans to be a subset of index scans, so that
> enable_indexscan should affect both.  I admit that the current set
> of GUCs doesn't let you force a bitmap scan over a plain one, but
> I can't recall many people complaining about that.  I don't follow
> the argument that this definition is somehow unmaintainable, either.

Well... but that's not what the GUC does either. Not now, and not with
the patch.

What happens right now is:

- If you set enable_indexscan=false, then disable_cost is added to the
cost of index scan paths and the cost of index-only scan paths.

- If you set enable_indexonlyscan=false, then index-only scan paths
are not generated at all.

Bitmap scans are controlled by enable_bitmapscan.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> What happens right now is:

> - If you set enable_indexscan=false, then disable_cost is added to the
> cost of index scan paths and the cost of index-only scan paths.

> - If you set enable_indexonlyscan=false, then index-only scan paths
> are not generated at all.

Hm.  The first part of that seems pretty weird to me --- why don't
we simply not generate the paths at all?  There is no case AFAIR
where that would prevent us from generating a valid plan.

(I do seem to recall that index-only paths are built on top of regular
index paths, so that there might be implementation issues with trying
to build the former and not the latter.  But you've probably looked
at that far more recently than I.)

            regards, tom lane

On Tue, Jul 2, 2024 at 2:37 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > What happens right now is:
>
> > - If you set enable_indexscan=false, then disable_cost is added to the
> > cost of index scan paths and the cost of index-only scan paths.
>
> > - If you set enable_indexonlyscan=false, then index-only scan paths
> > are not generated at all.
>
> Hm.  The first part of that seems pretty weird to me --- why don't
> we simply not generate the paths at all?  There is no case AFAIR
> where that would prevent us from generating a valid plan.

Well, yeah.

What the patch does is: if you set either enable_indexscan=false or
enable_indexonlyscan=false, then the corresponding path type is not
generated, and the other is unaffected. To me, that seems like the
logical way to clean this up.

One could argue for other things, of course. And maybe those other
things are fine, if they're properly justified and documented.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> What the patch does is: if you set either enable_indexscan=false or
> enable_indexonlyscan=false, then the corresponding path type is not
> generated, and the other is unaffected. To me, that seems like the
> logical way to clean this up.

> One could argue for other things, of course. And maybe those other
> things are fine, if they're properly justified and documented.

[ shrug... ]  This isn't a hill that I'm prepared to die on.
But I see no good reason to change the very long-standing
behaviors of these GUCs.

            regards, tom lane

On Tue, Jul 2, 2024 at 3:36 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > One could argue for other things, of course. And maybe those other
> > things are fine, if they're properly justified and documented.
>
> [ shrug... ]  This isn't a hill that I'm prepared to die on.
> But I see no good reason to change the very long-standing
> behaviors of these GUCs.

Well, I don't really know where to go from here. I mean, I think that
three committers (David, Heikki, yourself) have expressed some
concerns about changing the behavior. So maybe we shouldn't. But I
don't understand how it's reasonable to have two very similarly named
GUCs behave (1) inconsistently with each other and (2) in a way that
cannot be guessed from the documentation.

I feel like we're just clinging to legacy behavior on the theory that
somebody, somewhere might be relying on it in some way, which they
certainly might be. But that doesn't seem like a great reason, either.

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> Well, I don't really know where to go from here. I mean, I think that
> three committers (David, Heikki, yourself) have expressed some
> concerns about changing the behavior. So maybe we shouldn't. But I
> don't understand how it's reasonable to have two very similarly named
> GUCs behave (1) inconsistently with each other and (2) in a way that
> cannot be guessed from the documentation.

If the documentation isn't adequate, that's certainly an improvable
situation.  It doesn't seem hard:

-        Enables or disables the query planner's use of index-scan plan
-        types. The default is <literal>on</literal>.
+        Enables or disables the query planner's use of index-scan plan
+        types (including index-only scans).
+        The default is <literal>on</literal>.

More to the point, if we do change the longstanding meaning of this
GUC, that will *also* require documentation work IMO.

            regards, tom lane

On 02/07/2024 22:54, Robert Haas wrote:
> On Tue, Jul 2, 2024 at 3:36 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> One could argue for other things, of course. And maybe those other
>>> things are fine, if they're properly justified and documented.
>>
>> [ shrug... ]  This isn't a hill that I'm prepared to die on.
>> But I see no good reason to change the very long-standing
>> behaviors of these GUCs.
> 
> Well, I don't really know where to go from here. I mean, I think that
> three committers (David, Heikki, yourself) have expressed some
> concerns about changing the behavior. So maybe we shouldn't. But I
> don't understand how it's reasonable to have two very similarly named
> GUCs behave (1) inconsistently with each other and (2) in a way that
> cannot be guessed from the documentation.
> 
> I feel like we're just clinging to legacy behavior on the theory that
> somebody, somewhere might be relying on it in some way, which they
> certainly might be. But that doesn't seem like a great reason, either.

I agree the status quo is weird too. I'd be OK to break 
backwards-compatibility if we can make it better.

Tom mentioned enable_bitmapscan, and it reminded me that the current 
behavior with that is actually a bit annoying. I go through this pattern 
very often when I'm investigating query plans:

1. Hmm, let's see what this query plan looks like:

postgres=# explain analyze select * from foo where i=10;
                                                    QUERY PLAN 

----------------------------------------------------------------------------------------------------------------
  Index Scan using foo_i_idx on foo  (cost=0.29..8.31 rows=1 width=36) 
(actual time=0.079..0.090 rows=2 loops=1)
    Index Cond: (i = 10)
  Planning Time: 2.220 ms
  Execution Time: 0.337 ms
(4 rows)

2. Ok, and how long would it take with a seq scan? Let's see:

postgres=# set enable_indexscan=off;
SET
postgres=# explain analyze select * from foo where i=10;
                                                     QUERY PLAN 

------------------------------------------------------------------------------------------------------------------
  Bitmap Heap Scan on foo  (cost=4.30..8.31 rows=1 width=36) (actual 
time=0.102..0.113 rows=2 loops=1)
    Recheck Cond: (i = 10)
    Heap Blocks: exact=2
    ->  Bitmap Index Scan on foo_i_idx  (cost=0.00..4.30 rows=1 width=0) 
(actual time=0.067..0.068 rows=2 loops=1)
          Index Cond: (i = 10)
  Planning Time: 0.211 ms
  Execution Time: 0.215 ms
(7 rows)

3. Oh right, bitmap scan, I forgot about that one. Let's disable that too:

postgres=# set enable_bitmapscan=off;
SET
postgres=# explain analyze select * from foo where i=10;
                                             QUERY PLAN 

--------------------------------------------------------------------------------------------------
  Seq Scan on foo  (cost=0.00..1862.00 rows=1 width=36) (actual 
time=0.042..39.226 rows=2 loops=1)
    Filter: (i = 10)
    Rows Removed by Filter: 109998
  Planning Time: 0.118 ms
  Execution Time: 39.272 ms
(5 rows)

I would be somewhat annoyed if we add another step to that, to also 
disable index-only scans separately. It would be nice if 
enable_indexscan=off would also disable bitmap scans, that would 
eliminate one step from the above. Almost always when I want to disable 
index scans, I really want to disable the use of the index altogether. 
The problem then of course is, how do you force a bitmap scan without 
allowing other index scans, when you want to test them both?

It almost feels like we should have yet another GUC to disable index 
scans, index-only scans and bitmap index scans. "enable_indexes=off" or 
something.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Heikki Linnakangas <hlinnaka@iki.fi> writes:
> 3. Oh right, bitmap scan, I forgot about that one. Let's disable that too:

Yeah, I've hit that too, although more often (for me) it's the first
choice of plan.  In any case, it usually takes more than one change
to get to a seqscan.

> It almost feels like we should have yet another GUC to disable index 
> scans, index-only scans and bitmap index scans. "enable_indexes=off" or 
> something.

There's something to be said for that idea.  Breaking compatibility is
a little easier to stomach if there's a clear convenience win, and
this'd offer that.

            regards, tom lane

On Tue, Jul 2, 2024 at 5:39 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> I would be somewhat annoyed if we add another step to that, to also
> disable index-only scans separately. It would be nice if
> enable_indexscan=off would also disable bitmap scans, that would
> eliminate one step from the above. Almost always when I want to disable
> index scans, I really want to disable the use of the index altogether.
> The problem then of course is, how do you force a bitmap scan without
> allowing other index scans, when you want to test them both?
>
> It almost feels like we should have yet another GUC to disable index
> scans, index-only scans and bitmap index scans. "enable_indexes=off" or
> something.

This is an interesting idea, and it seems like it could be convenient.
However, the fact that it's so non-orthogonal is definitely not great.
One problem I've had with going through regression tests that rely on
the enable_* GUCs is that it's often not quite clear what values all
of those GUCs have at a certain point in the test file, because the
statements that set them may be quite a bit higher up in the file and
some changes may also have been rolled back. I've found recently that
the addition of EXPLAIN (SETTINGS) helps with this quite a bit,
because you can adjust the .sql file to use that option and then see
what shows up in the output file. Still, it's annoying, and the same
issue could occur in any other situation where you're using these
GUCs. It's just more confusing when there are multiple ways of turning
something off.

Would we consider merging enable_indexscan, enable_indexonlyscan, and
enable_bitmapscan into something like:

enable_indexes = on | off | { plain | indexonly | bitmap } [, ...]

I feel like that would solve the usability concern that you raise here
while also (1) preserving orthogonality and (2) reducing the number of
GUCs rather than first increasing it. When I first joined the project
there were a decent number of enable_* GUCs, but there's way more now.
Some of them are a little random (which is a conversation for another
day) but just cutting down on the number seems like it might not be
such a bad idea.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Wed, 3 Jul 2024 at 09:49, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Heikki Linnakangas <hlinnaka@iki.fi> writes:
> > 3. Oh right, bitmap scan, I forgot about that one. Let's disable that too:
>
> Yeah, I've hit that too, although more often (for me) it's the first
> choice of plan.  In any case, it usually takes more than one change
> to get to a seqscan.

I commonly hit this too.

I think the current behaviour is born out of the fact that we don't
produce both an Index Scan and an Index Only Scan for the same index.
We'll just make the IndexPath an index only scan, if possible based
on:

index_only_scan = (scantype != ST_BITMAPSCAN &&
   check_index_only(rel, index));

The same isn't true for Bitmap Index Scans.  We'll create both
IndexPaths and BitmapHeapPaths and let them battle it out in
add_path().

I suspect this is why it's been coded that enable_indexscan also
disables Index Only Scans.  Now, of course, it could work another way,
but I also still think that doing so is changing well-established
behaviour that I don't recall anyone ever complaining about besides
Robert. Robert's complaint seems to have originated from something he
noticed while hacking on code rather than actually using the database
for something. I think the argument for changing it should have less
weight due to that.

I understand that we do have inconsistencies around this stuff.  For
example, enable_sort has no influence on Incremental Sorts like
enable_indexscan has over Index Only Scan.  That might come from the
fact that we used to, up until a couple of releases ago, produce both
sort path types and let them compete in add_path().  That's no longer
the case, we now just do incremental sort when we can, just like we do
Index Only Scans when we can. Despite those inconsistencies, I
wouldn't vote for changing either of them to align with the other. It
just feels too long-established behaviour to be messing with.

I feel it might be best to move this patch to the back of the series
or just drop it for now as it seems to be holding up the other stuff
from moving forward, and that stuff looks useful and worth changing.

David

OK, here's a new patch version. I earlier committed the refactoring to
avoid using disable_cost to force WHERE CURRENT OF to be implemented
by a TID scan. In this version, I've dropped everything related to
reworking enable_indexscan or any other enable_* GUC. Hence, this
version of the patch set just focuses on adding the count of disabled
nodes and removing the use of disable_cost. In addition to dropping
the controversial patches, I've also found and squashed a few bugs in
this version.

Behavior: With the patch, whenever an enable_* GUC would cause
disable_cost to be added, disabled_nodes is incremented instead. There
is one remaining use of disable_cost which is not triggered by an
enable_* GUC but by the desire to avoid plans that we think will
overflow work_mem. I welcome thoughts on what to do about that case;
for now, I do nothing. As before, 0001 adds the disabled_nodes field
to paths and 0002 adds it to plans. I think we could plausibly commit
only 0001, both patches separately, or both patches squashed.

Notes:

- I favor committing both patches. Tom stated that he didn't think
that we needed to show anything related to disabled nodes, and that
could be true. However, today, you can tell which nodes are disabled
as long as you print out the costs; if we don't propagate disabled
nodes into the plan and print them out, that will no longer be
possible. I found working on the patches that it was really hard to
debug the patch set without this, so my guess is that we'll find not
having it pretty annoying, but we can also just commit 0001 for
starters and see how long it takes for the lack of 0002 to become
annoying. If the answer is "infinite time," that's cool; if it isn't,
we can reconsider committing 0002.

- If we do commit 0002, I think it's a good idea to have the number of
disabled nodes displayed even with COSTS OFF, because it's stable, and
it's pretty useful to be able to see this in the regression output. I
have found while working on this that I often need to adjust the .sql
files to say EXPLAIN (COSTS ON) instead of EXPLAIN (COSTS OFF) in
order to understand what's happening. Right now, there's no real
alternative because costs aren't stable, but disabled-node counts
should be stable, so I feel this would be a step forward. Apart from
that, I also think it's good for features to have regression test
coverage, and since we use COSTS OFF everywhere or at least nearly
everywhere in the regression test, if we don't print out the disabled
node counts when COSTS OFF is used, then we don't cover that case in
our tests. Bummer.

Regression test changes in 0001:

- btree_index.sql executes a query "select proname from pg_proc where
proname ilike 'ri%foo' order by 1" with everything but bitmap scans
disabled. Currently, that produces an index-only scan; with the patch,
it produces a sort over a sequential scan. That's a little odd,
because the test seems to be aimed at demonstrating that we can use a
bitmap scan, and it doesn't, because we apparently can't. But, why
does the patch change the plan?
At least on my machine, the index-only scan is significantly more
costly than the sequential scan. I think what's happening here is that
when you add disable_cost to the cost of both paths, they compare
fuzzily the same; without that, the cheaper one wins.

- select_parallel.out executes a query with sequential scans disabled
but tenk2 must nevertheless be sequential-scanned. With the patch,
that changes to a parallel sequential scan. I think the explanation
here is the same as in the preceding case.

- horizons.spec currently sets enable_seqscan=false,
enable_indexscan=false, and enable_bitmapscan=false. I suspect that
Andres thought that this would force the use of an index-only scan,
since nothing sets enable_indexonlyscan=false. But as discussed
upthread, that is not true. Instead everything is disabled. For the
same reasons as in the previous two examples, this caused an
assortment of plan changes which in turn caused the test to fail to
test what it was intended to test. So I removed enable_indexscan=false
from the spec file, and now it gets index-only scans everywhere again,
as desired.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

On Thu, 1 Aug 2024 at 04:23, Robert Haas <robertmhaas@gmail.com> wrote:
> OK, here's a new patch version.

I think we're going down the right path here.

I've reviewed both patches, here's what I noted down during my review:

0. I've not seen any mention so far about postgres_fdw's
use_remote_estimate.  Maybe changing the costs is fixing an issue that
existed before. I'm just not 100% sure on that.

Consider:

CREATE EXTENSION postgres_fdw;

DO $d$
    BEGIN
        EXECUTE $$CREATE SERVER loopback FOREIGN DATA WRAPPER postgres_fdw
            OPTIONS (use_remote_estimate 'true',
                     dbname '$$||current_database()||$$',
                     port '$$||current_setting('port')||$$'
            )$$;
    END;
$d$;
CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;

create table t (a int);
create foreign table ft (a int) server loopback OPTIONS (table_name 't');

alter system set enable_seqscan=0;
select pg_Reload_conf();
set enable_seqscan=1;
explain select * from ft;

patched:
 Foreign Scan on ft  (cost=100.00..671.00 rows=2550 width=4)

master:
 Foreign Scan on ft  (cost=10000000100.00..10000000671.00 rows=2550 width=4)

I kinda think that might be fixing an issue that I don't recall being
reported before. I think we shouldn't really care that much about what
nodes are disabled on the remote server and not having disabled_cost
applied to that gives us that.

1. The final sentence of the function header comment needs to be
updated in estimate_path_cost_size().

2. Does cost_tidscan() need to update the header comment to say
tidquals must not be empty?

3. final_cost_nestloop() seems to initially use the disabled_nodes
from initial_cost_nestloop() but then it goes off and calculates it
again itself. One of these seems redundant. The "We could include
disable_cost in the preliminary estimate" comment explains why it was
originally left to final_cost_nestloop(), so maybe worth sticking to
that? I don't quite know the full implications, but it does not seem
worth risking a behaviour change here.

4. I wonder if it's worth doing a quick refactor of the code in
initial_cost_mergejoin() to get rid of the duplicate code in the "if
(outersortkeys)" and "if (innersortkeys)" branches. It seems ok to do
outer_path = &sort_path. Likewise for inner_path.

5. final_cost_hashjoin() does the same thing as #3

6. createplan.c adds #include "nodes/print.h" but doesn't seem to add
any code that might use anything in there.

7. create_lockrows_path() needs to propagate disabled_nodes.

create table a (a int);
set enable_seqscan=0;

explain select * from a for update limit 1;

 Limit  (cost=0.00..0.02 rows=1 width=10)
   ->  LockRows  (cost=0.00..61.00 rows=2550 width=10)
         ->  Seq Scan on a  (cost=0.00..35.50 rows=2550 width=10)
               Disabled Nodes: 1
(4 rows)


explain select * from a limit 1;

 Limit  (cost=0.00..0.01 rows=1 width=4)
   Disabled Nodes: 1
   ->  Seq Scan on a  (cost=0.00..35.50 rows=2550 width=4)
         Disabled Nodes: 1
(4 rows)

8. There's something weird with CTEs too.

create table b(a int);
set enable_sort=0;

Patched:

explain with cte as materialized (select * from b order by a) select *
from cte order by a desc;

 Sort  (cost=381.44..387.82 rows=2550 width=4)
   Disabled Nodes: 1
   Sort Key: cte.a DESC
   CTE cte
     ->  Sort  (cost=179.78..186.16 rows=2550 width=4)
           Disabled Nodes: 1
           Sort Key: b.a
           ->  Seq Scan on b  (cost=0.00..35.50 rows=2550 width=4)
   ->  CTE Scan on cte  (cost=0.00..51.00 rows=2550 width=4)
(9 rows)

master:

explain with cte as materialized (select * from a order by a) select *
from cte order by a desc;

 Sort  (cost=20000000381.44..20000000387.82 rows=2550 width=4)
   Sort Key: cte.a DESC
   CTE cte
     ->  Sort  (cost=10000000179.78..10000000186.16 rows=2550 width=4)
           Sort Key: a.a
           ->  Seq Scan on a  (cost=0.00..35.50 rows=2550 width=4)
   ->  CTE Scan on cte  (cost=0.00..51.00 rows=2550 width=4)
(7 rows)

I'd expect the final sort to have disabled_nodes == 2 since
disabled_cost has been added twice in master.

9. create_set_projection_path() needs to propagate disabled_nodes too:

explain select b from (select a,generate_series(1,2) as b from b) a  limit 1;

 Limit  (cost=0.00..0.03 rows=1 width=4)
   ->  Subquery Scan on a  (cost=0.00..131.12 rows=5100 width=4)
         ->  ProjectSet  (cost=0.00..80.12 rows=5100 width=8)
               ->  Seq Scan on b  (cost=0.00..35.50 rows=2550 width=0)
                     Disabled Nodes: 1

10. create_setop_path() needs to propagate disabled_nodes.

explain select * from b except select * from b limit 1;

 Limit  (cost=0.00..0.80 rows=1 width=8)
   ->  HashSetOp Except  (cost=0.00..160.25 rows=200 width=8)
         ->  Append  (cost=0.00..147.50 rows=5100 width=8)
               Disabled Nodes: 2
               ->  Subquery Scan on "*SELECT* 1"  (cost=0.00..61.00
rows=2550 width=8)
                     Disabled Nodes: 1
                     ->  Seq Scan on b  (cost=0.00..35.50 rows=2550 width=4)
                           Disabled Nodes: 1
               ->  Subquery Scan on "*SELECT* 2"  (cost=0.00..61.00
rows=2550 width=8)
                     Disabled Nodes: 1
                     ->  Seq Scan on b b_1  (cost=0.00..35.50 rows=2550 width=4)
                           Disabled Nodes: 1
(12 rows)

11. create_modifytable_path() needs to propagate disabled_nodes.

explain with cte as (update b set a = a+1 returning *) select * from
cte limit 1;

 Limit  (cost=41.88..41.90 rows=1 width=4)
   CTE cte
     ->  Update on b  (cost=0.00..41.88 rows=2550 width=10)
           ->  Seq Scan on b  (cost=0.00..41.88 rows=2550 width=10)
                 Disabled Nodes: 1
   ->  CTE Scan on cte  (cost=0.00..51.00 rows=2550 width=4)
(6 rows)

12. For the 0002 patch, I do agree that having this visible in EXPLAIN
is a must.  I'd much rather see:  Disabled: true/false.  And just
display this when the disabled_nodes is greater than the sum of the
subpaths.  That might be much more complex to implement, but it's
going to make it much easier to track down the disabled nodes in very
large plans.

David

On Wed, Jul 31, 2024 at 10:01 PM David Rowley <dgrowleyml@gmail.com> wrote:
> I've reviewed both patches, here's what I noted down during my review:

Thanks.

> 0. I've not seen any mention so far about postgres_fdw's
> use_remote_estimate.  Maybe changing the costs is fixing an issue that
> existed before. I'm just not 100% sure on that.
>
> patched:
>  Foreign Scan on ft  (cost=100.00..671.00 rows=2550 width=4)
>
> master:
>  Foreign Scan on ft  (cost=10000000100.00..10000000671.00 rows=2550 width=4)
>
> I kinda think that might be fixing an issue that I don't recall being
> reported before. I think we shouldn't really care that much about what
> nodes are disabled on the remote server and not having disabled_cost
> applied to that gives us that.

Hmm, I think it's subjective which behavior is better. If somebody
thought the new behavior was worse, they might want the remote side's
count of disabled nodes to be propagated to the local side, but I'm
disinclined to go there. My guess is that it doesn't matter much
either way what we do here, so I'd rather not add more code.

> 1. The final sentence of the function header comment needs to be
> updated in estimate_path_cost_size().

Fixed.

> 2. Does cost_tidscan() need to update the header comment to say
> tidquals must not be empty?

IMHO, no. The assertions I added to that function were intended as
documentation of what that function was already assuming about the
behavior of its caller. I had to trace through the logic in tidpath.c
for quite a while to understand why cost_tidscan() was not completely
broken. To spare the next person the trouble of working that out, I
added assertions. Now we could additionally add commentary in English
that restates what the assertions already say, but I feel like having
the assertions is good enough. If somebody ever whacks around
tidpath.c such that these assertions start failing, I think it will be
fairly clear to them that they either need to revert their changes in
tidpath.c or upgrade the logic in this function to cope.

> 3. final_cost_nestloop() seems to initially use the disabled_nodes
> from initial_cost_nestloop() but then it goes off and calculates it
> again itself. One of these seems redundant.

Oops. Fixed.

> The "We could include
> disable_cost in the preliminary estimate" comment explains why it was
> originally left to final_cost_nestloop(), so maybe worth sticking to
> that? I don't quite know the full implications, but it does not seem
> worth risking a behaviour change here.

I don't really see how there could be a behavior change here, unless
there's a bug. Dealing with the enable_* flags in initial_cost_XXX
rather than final_cost_XXX could be better or worse from a performance
standpoint and it could make for cleaner or less clean code, but the
user-facing behavior should be identical unless there are bugs.

The reason why I changed this is because of the logic in
add_path_precheck(): it exits early as soon as it sees a path whose
total cost is greater than the cost of the proposed new path. Since
the patch's aim is to treat disabled_node as a high-order component of
the cost, we need to make the same decision by comparing the count of
disabled_nodes first and then if that is equal, we need to compare the
total_cost. We can't do that if we don't have the count of
disabled_nodes for the proposed new path.

I think this may be a bit hard to understand, so let me give a
concrete example. Suppose we're planning some join where one side can
only be planned with a sequential scan and sequential scans are
disabled. We have ten paths in the path list and they have costs of
1e10+100, 1e10+200, ..., 1e10+1000. Now add_path_precheck() is asked
to consider a new path where there is a disabled node on BOTH sides of
the join -- the one side has the disabled sequential scan, but now the
other side also has something disabled, so the cost is let's say
2e10+79. add_path_precheck() can see at once that this path is a
loser: it can't possibly dominate any path that already exists,
because it costs more than any of them. But when you take disable_cost
out, things look quite different. Now you have a proposed path with a
total_cost of 79 and a path list with costs of 100, ..., 1000. If
you're not allowed to know anything about disabled_nodes, the new path
looks like it might be valuable. You might decide to construct it and
try inserting into the pathlist, which will end up being useless, and
even if you don't, you're going to compare its pathkeys and
parameterization to each of the 10 existing paths before giving up.
Bummer.

So, to avoid getting much stupider than it is currently,
add_path_precheck() needs a preliminary estimate of the number of
disabled nodes just like it needs a preliminary estimate of the total
cost. And to avoid regressions, that estimate needs to be pretty good.
A naive estimate would be to just add up the number of disabled_nodes
on the inner and outer paths, but that would be a regression in the
merge-join case, because initial_cost_mergejoin() calls cost_sort()
for the inner and outer sides and that will add disable_cost if sorts
are disabled. If you didn't take the effect of cost_sort() into
account, you might think that your number of disabled_nodes was going
to be substantially lower than it really would be, leading to wasted
work as described in the last paragraph. Plus, since
initial_cost_mergejoin() is incurring the overhead of calling
cost_sort() anyway to get the total cost numbers anyway, it would be
silly not to save the count of disabled nodes: if we did, we'd have to
redo the cost_sort() call in final_cost_mergejoin(), which would be
expensive.

If we wanted to make our estimate of the # of disabled nodes exactly
comparable to what we now do with disable_cost, we would postpone if
(!enable_WHATEVERjoin) ++disabled_nodes to the final_cost_XXX
functions and do all of the other accounting related to disabled nodes
at the initial_cost_XXX phase. But I do not like that approach.
Postponing one trivial portion of the disabled_nodes calculation to a
later time won't save any significant number of CPU cycles, but it
might confuse people reading the code. You then have to know that the
disabled_nodes count that gets passed to final_cost_XXX is not yet the
final count, but that you may still need to add 1 for the join itself
(but not for the implicit sorts that the join requires, which have
already been accounted for). That's the kind of odd definition that
breeds bugs. Besides, it's not as if moving that tiny bit of logic to
the initial_cost_XXX functions has no upside: it could allow
add_path_precheck() to exit earlier, thus saving cycles.

(For the record, the explanation above took about 3 hours to write, so
I hope it's managed to be both correct and convincing. This stuff is
really complicated.)

> 4. I wonder if it's worth doing a quick refactor of the code in
> initial_cost_mergejoin() to get rid of the duplicate code in the "if
> (outersortkeys)" and "if (innersortkeys)" branches. It seems ok to do
> outer_path = &sort_path. Likewise for inner_path.

I don't think that's better.

> 5. final_cost_hashjoin() does the same thing as #3

Argh. Fixed.

> 6. createplan.c adds #include "nodes/print.h" but doesn't seem to add
> any code that might use anything in there.

Fixed.

> 8. There's something weird with CTEs too.
>
> I'd expect the final sort to have disabled_nodes == 2 since
> disabled_cost has been added twice in master.

Right now, disabled node counts don't propagate through SubPlans (see
SS_process_ctes). Maybe that needs to be changed, but aside from
looking weird, does it do any harm?

> 7. create_lockrows_path() needs to propagate disabled_nodes.
> 9. create_set_projection_path() needs to propagate disabled_nodes too:
> 10. create_setop_path() needs to propagate disabled_nodes.
> 11. create_modifytable_path() needs to propagate disabled_nodes.

I changed all of these, but I think these examples only establish that
those nodes DO NOT propagate disabled_nodes, not that they need to. If
we're past the point of making any choices based on costs, then
maintaining disabled_nodes or not doing so won't affect correctness.
That's not to say these aren't good to tidy up, and some of them may
well be bugs, but I don't think your test cases prove that. What
primarily matters is whether the enable_BLAH GUCs get respected; the
exact contents of the EXPLAIN output are somewhat more arguable.

> 12. For the 0002 patch, I do agree that having this visible in EXPLAIN
> is a must.  I'd much rather see:  Disabled: true/false.  And just
> display this when the disabled_nodes is greater than the sum of the
> subpaths.  That might be much more complex to implement, but it's
> going to make it much easier to track down the disabled nodes in very
> large plans.

I think it's going to be very unpleasant if we have the planner add
things up and then try to have EXPLAIN subtract them back out again.
One problem with that is that all of the test cases where you just
showed disabled_nodes not propagating upward wouldn't actually show
anything any more, because disabled_nodes would not have been greater
in the parent than in the child. So those are oversights in the code
that are easy to spot now but would become hard to spot with this
implementation. Another problem is that the EXPLAIN code itself could
contain bugs, or slightly more broadly, get out of sync with the logic
that decides what to add up. It won't be obvious what's happening:
some node that is actually disabled just won't appear to be, or the
other way around, and it will be hard to understand what happened,
because you won't be able to see the raw counts of disabled nodes that
would allow you to deduce where the error actually is.

One idea that occurs to me is to store TWO counts in each path node
and each plan node: the count of self-exclusive disabled nodes, and
the count of self-include disabled nodes. Then explain can just test
if they are different. If the answer is 1, the node is disabled; if 0,
it's enabled; if anything else, there's a bug (and it could print the
delta, or each value separately, to help localize such bugs). The
problem with that is that it eats up more space in
performance-critical data structures, but perhaps that's OK: I don't
know.

Another thought is that right now you just see the disable_cost values
added up with the rest of the cost. So maybe propagating upward is not
really such a bad behavior; it's what we have now.

This point probably needs more thought and discussion, but I'm out of
time to work on this for today, and out of mental energy too. So for
now here's v5 as I have it.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Fri, 2 Aug 2024 at 06:03, Robert Haas <robertmhaas@gmail.com> wrote:
> I think this may be a bit hard to understand, so let me give a
> concrete example. Suppose we're planning some join where one side can
> only be planned with a sequential scan and sequential scans are
> disabled. We have ten paths in the path list and they have costs of
> 1e10+100, 1e10+200, ..., 1e10+1000. Now add_path_precheck() is asked
> to consider a new path where there is a disabled node on BOTH sides of
> the join -- the one side has the disabled sequential scan, but now the
> other side also has something disabled, so the cost is let's say
> 2e10+79. add_path_precheck() can see at once that this path is a
> loser: it can't possibly dominate any path that already exists,
> because it costs more than any of them. But when you take disable_cost
> out, things look quite different. Now you have a proposed path with a
> total_cost of 79 and a path list with costs of 100, ..., 1000. If
> you're not allowed to know anything about disabled_nodes, the new path
> looks like it might be valuable. You might decide to construct it and
> try inserting into the pathlist, which will end up being useless, and
> even if you don't, you're going to compare its pathkeys and
> parameterization to each of the 10 existing paths before giving up.
> Bummer.

OK, so it sounds like you'd like to optimise this code so that the
planner does a little less work when node types are disabled. The
existing comment does mention explicitly that we don't want to do
that:

/*
* We could include disable_cost in the preliminary estimate, but that
* would amount to optimizing for the case where the join method is
* disabled, which doesn't seem like the way to bet.
*/

As far as I understand it from reading the comments in that file, I
see no offer of guarantees that the initial cost will be cheaper than
the final cost. So what you're proposing could end up rejecting paths
based on initial cost where the final cost might end up being the
cheapest path. Imagine you're considering a Nested Loop and a Hash
Join, both of which are disabled. Merge Join is unavailable as the
join column types are not sortable.  If the hash join costs 99 and the
initial nested loop costs 110, but the final nested loop ends up
costing 90, then the nested loop could be rejected before we even get
to perform the final cost for it. The current code will run
final_cost_nestloop() and find that 90 is cheaper than 99, whereas
what you want to do is stop bothering with nested loop when we see the
initial cost come out at 110.

Perhaps it's actually fine if the initial costs are always less than
the final costs as, if that's the case, we won't ever reject any paths
based on the initial cost that we wouldn't anyway based on the final
cost. However, since there does not seem to be any comments mentioning
this guarantee and if you're just doing this to squeeze more
performance out of the planner, it seems risky to do for that reason
alone.

I'd say if you want to do this, you should be justifying it on its own
merit with some performance numbers and some evidence that we don't
produce inferior plans as a result. But per what I quoted above,
you're not doing that, you're doing this as a performance
optimisation.

I'm not planning on pushing this any further. I've just tried to
highlight that there's the possibility of a behavioural change. You're
claiming there isn't one. I claim there is.

David

On Thu, Aug 1, 2024 at 11:34 PM David Rowley <dgrowleyml@gmail.com> wrote:
> I'm not planning on pushing this any further. I've just tried to
> highlight that there's the possibility of a behavioural change. You're
> claiming there isn't one. I claim there is.

I don't know what to tell you. The original version of the patch
didn't change this stuff, and the result did not work. So I looked
into the problem and fixed it. I may have done that wrongly, or there
may be debatable points, but it seems like your argument is
essentially that I shouldn't have done any of this and I should just
take it all back out, and I know that doesn't work because it's the
first thing I tried.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Sat, 3 Aug 2024 at 00:17, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Thu, Aug 1, 2024 at 11:34 PM David Rowley <dgrowleyml@gmail.com> wrote:
> > I'm not planning on pushing this any further. I've just tried to
> > highlight that there's the possibility of a behavioural change. You're
> > claiming there isn't one. I claim there is.
>
> I don't know what to tell you. The original version of the patch
> didn't change this stuff, and the result did not work. So I looked
> into the problem and fixed it. I may have done that wrongly, or there
> may be debatable points, but it seems like your argument is
> essentially that I shouldn't have done any of this and I should just
> take it all back out, and I know that doesn't work because it's the
> first thing I tried.

I've just read what you wrote again and I now realise something I didn't before.

I now think neither of us got it right. I now think what you'd need to
do to be aligned to the current behaviour is have
initial_cost_nestloop() add the disabled_nodes for the join's subnodes
*only* and have final_cost_nestloop() add the additional
disabled_nodes if enable_nestloop = off. That way you maintain the
existing behaviour of not optimising for disabled node types and don't
risk plan changes if the final cost comes out cheaper than the initial
cost.

David

On Fri, Aug 2, 2024 at 9:13 AM David Rowley <dgrowleyml@gmail.com> wrote:
> I now think neither of us got it right. I now think what you'd need to
> do to be aligned to the current behaviour is have
> initial_cost_nestloop() add the disabled_nodes for the join's subnodes
> *only* and have final_cost_nestloop() add the additional
> disabled_nodes if enable_nestloop = off. That way you maintain the
> existing behaviour of not optimising for disabled node types and don't
> risk plan changes if the final cost comes out cheaper than the initial
> cost.

All three initial_cost_XXX functions have a comment that says "This
must quickly produce lower-bound estimates of the path's startup and
total costs," i.e. the final cost should never be cheaper. I'm pretty
sure that it was the design intention here that no path ever gets
rejected at the initial cost stage that would have been accepted at
the final cost stage.

(You can also see, as a matter of implementation, that they extract
the startup_cost and run_cost from the workspace and then add to those
values.)

--
Robert Haas
EDB: http://www.enterprisedb.com

Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Aug 2, 2024 at 9:13 AM David Rowley <dgrowleyml@gmail.com> wrote:
>> ... That way you maintain the
>> existing behaviour of not optimising for disabled node types and don't
>> risk plan changes if the final cost comes out cheaper than the initial
>> cost.

> All three initial_cost_XXX functions have a comment that says "This
> must quickly produce lower-bound estimates of the path's startup and
> total costs," i.e. the final cost should never be cheaper. I'm pretty
> sure that it was the design intention here that no path ever gets
> rejected at the initial cost stage that would have been accepted at
> the final cost stage.

That absolutely is the expectation, and we'd better be careful not
to break it.

            regards, tom lane

On Fri, Aug 2, 2024 at 12:51 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> That absolutely is the expectation, and we'd better be careful not
> to break it.

I have every intention of not breaking it. :-)

--
Robert Haas
EDB: http://www.enterprisedb.com