Re: pg9.6 segfault using simple query (related to use fk for join estimates)

Hi,

On 05/04/2016 08:54 PM, Tom Lane wrote:
> David Rowley <david.rowley@2ndquadrant.com> writes:
>> On 4 May 2016 at 09:18, David Rowley <david.rowley@2ndquadrant.com> wrote:
>>> On 4 May 2016 at 02:10, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
>>>> There are probably a few reasonably simple things we could do - e.g. ignore
>>>> foreign keys with just a single column, as the primary goal of the patch is
>>>> improving estimates with multi-column foreign keys. I believe that covers a
>>>> vast majority of foreign keys in the wild.
>
>> I've spent a few hours looking at this and I've come up with the
>> attached patch, which flags each ForeignKeyOptInfo to say whether its
>> possible to be referenced in any join condition, with the logic that
>> if the referenced relation is in the simple_rte_array, then it could
>> be referenced.
>
> My fundamental problem with the FK-selectivity patch is that it looks
> an awful lot like a preliminary proof-of-concept that got committed.
>
> I do not like the basic design: it's about as brute-force as could
> possibly be.  It adds code that is executed:
>
>   * at least once per join relation created (hence, significantly more than
>       the number of rels in the query; see also get_joinrel_parampathinfo)
>     * for each inner relation in the initial input joinrel pair
>       * for each outer relation in the initial joinrel pair
>         * for each foreign key constraint on this inner and outer rel
>           * for each key column in that FK
>             * for each join qual for the current input joinrel pair
>               * for each member of the relevant EquivalenceClass
>
> This is at least O(N^3) in the number of baserels in the query, not
> to mention the other multipliers. I'm not very impressed by tests
> that scale only one of the multipliers (like the number of FK
> constraints); where the pain is going to come in is when all of these
> factors are large.
>
> I spent some time trying to make a test case that was impossibly
> slow, without any really impressive result: I saw at most about a 25%
> growth in planning time, for a 27-way join with one or two foreign
> keys per table. I noted however that with a simple FROM list of
> tables, you don't really get the full force of the combinatorial
> explosion, because join_search_one_level prefers to generate
> left-deep trees first, and so the first creation of a given joinrel
> is always N left-side rels against 1 right-side rel, causing the
> second level of looping to always iterate just once. (GEQO behaves
> likewise, I think.) I spent a little time trying to devise join order
> constraints that would result in a lot of high-level joinrels being
> formed with many relations on both sides, which would cause both of
> the second and third levels to iterate O(N) times not just once. I
> didn't have much luck, but I have zero confidence that our users
> won't find such cases.

Don't know. We haven't found such extreme example either.

>
> The reason it's so brute force is that it's rediscovering the same
> facts over and over. In all simple (non-outer-join) cases, the only
> clauses that are of any interest are derived from EquivalenceClasses,
> and all that you really need to know is "are the vars mentioned on
> each side of this FK present in the same EquivalenceClass?". ISTM
> that could be determined once per FK per query and cached in or near
> the EC, not expensively rediscovered at each level of joining. I'm
> not sure whether it'd be worth considering non-EC-derived equalities
> (ie, outer join clauses) at all, and note that the current patch
> fails to do so anyway; see below.

I'm not sure it's that simple, as it also depends on the join order, so 
if we only detect that once per query we'll get incorrect estimates for 
the intermediate results. I think the approach with cache proposed by 
David few days ago is the best approach.

>
> My other design-level complaint is that basing this on foreign keys is
> fundamentally the wrong thing.  What actually matters is the unique index
> underlying the FK; that is, if we have "a.x = b.y" and there's a
> compatible unique index on b.y, we can conclude that no A row will match
> more than one B row, whether or not an explicit FK relationship has been
> declared.  So we should drive this off unique indexes instead of FKs,
> first because we will find more cases, and second because the planner
> already examines indexes and doesn't need any additional catalog lookups
> to get the required data.  (IOW, the relcache additions that were made in
> this patch series should go away too.)

No, that's not what the patch does, and it can't use unique indexes 
instead. The patch improves estimation with multi-column foreign keys, 
when the join matches the constraint. Currently we treat the conditions 
as independent and multiply the estimated selectivities, completely 
ignoring the guarantee provided by the FK, which leads to significant 
under-estimates.

So when you have:

CREATE TABLE t1 (a1 INT, a2 INT, primary key (a1,a2));
CREATE TABLE t2 (b1 INT, b2 INT,                 FOREIGN KEY (b1,b2) REFERENCES t1(a1,a2));

and do
   SELECT * FROM t1, t2 WHERE a1=b1 AND a2=b2;

the patch realizes that is should not multiply the selectivities.

But unique indexes are insufficient for this - it's the foreign key 
between the two tables that allows us to do this.

Consider this:

CREATE TABLE t1 (a1 INT, a2 INT, UNIQUE (a1,a2));
CREATE TABLE t2 (b1 INT, b2 INT);

INSERT INTO t1 SELECT i, i FROM generate_series(1,10000) s(i);
INSERT INTO t2 SELECT 10000*random(), 10000*random()                 FROM generate_series(1,10000) s(i);

and do the same query. In this case multiplying the selectivities is the 
right thing to do, as the unique index provides no guarantees.


>
> Aside from the design being basically wrong, the code quality seems pretty
> low.  Aside from the missing IsA check that started this thread, I found
> the following problems in a quick once-over of 137805f89:
>
> Bugs in quals_match_foreign_key():
>
> * Test for clause->opno == fkinfo->conpfeqop[i] fails to consider
> cross-type operators, ie, what's in the clause might be int2 = int4
> while the conpfeqop is int4 = int2.
>
> * parent_ec check isn't really the right thing, since EC-derived clauses
> might not have that set.  I think it may be adequate given that you only
> deal with simple Vars, but at least a comment about that would be good.
>
> * Come to think of it, you could probably dodge the commutator operator
> problem altogether for clauses with nonnull parent_ec, because they must
> contain a relevant equality operator.  (Although if it's redesigned as I
> suggest above, the code path for a clause with parent_ec set would look
> totally different from this anyway.)
>
> * Maintaining the fkmatches bitmapset is useless expense, just use a
> counter of matched keys.  Or for that matter, why not just fail
> immediately if i'th key is not found?
>
> find_best_foreign_key_quals():
>
> * Test on enable_fkey_estimates should be one call level up to dodge the
> useless doubly-nested loop in clauselist_join_selectivity (which would
> make the "fast path" exit here pretty much useless)
>
> clauselist_join_selectivity():
>
> * "either could be zero, but not both" is a pretty unhelpful comment given
> the if-test just above it.  What *could* have used some explanation is
> what the next two dozen lines are doing, because they're as opaque as can
> be; and the XXX comment doesn't leave a warm feeling that the author
> understands it either.  I'm not prepared to opine on whether this segment
> of code is correct at all without better commentary.
>
> calc_joinrel_size_estimate():
>
> * why is clauselist_join_selectivity applied to pushed-down clauses and
> not local ones in an outer join?  If that's not an oversight, it at least
> requires an explanatory comment.  Note that this means we are not applying
> FK knowledge for "a left join b on x = y", though it seems like we could.
>
> compute_semi_anti_join_factors isn't using clauselist_join_selectivity
> either.  I don't know whether it should be, but if not, a comment about
> why not seems appropriate.  More generally, I wonder why this logic
> wasn't just folded into clauselist_selectivity.
>
> guc.c:
> undocumented GUCs are not acceptable
>
> paths.h:
> patch introduces an extern that's referenced noplace
>
> In short, I'm not left with a warm fuzzy feeling about this patch having
> been ready to commit.  The predecessor patch 015e88942 was also
> underreviewed, cf 5306df283.

OK, thanks for the comments.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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