Обсуждение: Unwanted expression simplification in PG12b2

=?UTF-8?Q?Darafei_=22Kom=D1=8Fpa=22_Praliaskouski?= <me@komzpa.net> writes:
> Many thanks for the parallel improvements in Postgres 12. Here is one of
> cases where a costy function gets moved from a parallel worker into main
> one, rendering spatial processing single core once again on some queries.
> Perhaps an assumption "expressions should be mashed together as much as
> possible" should be reviewed and something along "biggest part of
> expression should be pushed down into parallel worker"?

I don't see anything in your test case that proves what you think it does.
The expensive calculation *is* being done in the worker in the first
example.  It's not real clear to me why the first example is only choosing
to use one worker rather than 3, but probably with a larger test case
(ie bigger table) that decision would change.

Just to clarify --- when you see something like this:

> │ Gather  (cost=159.86..42668.93 rows=200 width=36)
> │   Output: (st_collect(st_difference(postgis_test_table.a,st_unaryunion(postgis_test_table.b)))),
postgis_test_table.id
> │   ->  GroupAggregate  (cost=59.86..42568.73 rows=200 width=36)
> │         Output: st_collect(st_difference(postgis_test_table.a,st_unaryunion(postgis_test_table.b))),
postgis_test_table.id

EXPLAIN is trying to tell you that the expression value is being
computed by the lower plan node, and just passed up to the upper
plan node --- that's what the extra parens in the upper expression
printout mean.  Perhaps there's some way to make that clearer,
but I haven't thought of one that doesn't seem very clutter-y.

            regards, tom lane

On Wed, Jul 17, 2019 at 5:20 PM Darafei "Komяpa" Praliaskouski
<me@komzpa.net> wrote:
> Indeed, it seems I failed to minimize my example.
>
> Here is the actual one, on 90GB table with 16M rows:
> https://gist.github.com/Komzpa/8d5b9008ad60f9ccc62423c256e78b4c
>
> I can share the table on request if needed, but hope that plan may be enough.

[ replying to an old thread ]

I think that this boils down to a lack of planner smarts about target
lists. The planner currently assumes that any given relation - which
for planner purposes might be an actual table or might be the result
of joining multiple tables, aggregating something, running a subquery,
etc. - more or less has one thing that it's supposed to produce. It
only tries to generate plans that produce that target list. There's
some support in there for the idea that there might be various paths
for the same relation that produce different answers, but I don't know
of that actually being used anywhere (but it might be).

What I taught the planner to do here had to do with making the costing
more accurate for cases like this. It now figures out that if it's
going to stick a Gather in at that point, computing the expressions
below the Gather rather than above the Gather makes a difference to
the cost of that plan vs. other plans. However, it still doesn't
consider any more paths than it did before; it just costs them more
accurately. In your first example, I believe that the planner should
be able to consider both GroupAggregate -> Gather Merge -> Sort ->
Parallel Seq Scan and GroupAggregate -> Sort -> Gather -> Parallel Seq
Scan, but I think it's got a fixed idea about which fields should be
fed into the Sort. In particular, I believe it thinks that sorting
more data is so undesirable that it doesn't want to carry any
unnecessary baggage through the Sort for any reason. To solve this
problem, I think it would need to cost the second plan with projection
done both before the Sort and after the Sort and decide which one was
cheaper.

This class of problem is somewhat annoying in that the extra planner
cycles and complexity to deal with getting this right would be useless
for many queries, but at the same time, there are a few cases where it
can win big. I don't know what to do about that.

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

On Sun, Sep 22, 2019 at 7:47 AM Darafei "Komяpa" Praliaskouski
<me@komzpa.net> wrote:
> A heuristic I believe should help my case (and I hardly imagine how it can break others) is that in presence of
Gather,all the function calls that are parallel safe should be pushed into it. 

The cost of pushing data through the Sort is not necessarily
insignificant.  Your functions are (IIUC) extremely expensive, so it's
worth going to any length to reduce the time spent evaluating them.
However, if someone has ||(text,text) in the tlist, that might be the
wrong approach, because it's not saving much to compute that earlier
and it might make the sort a lot wider, especially if de-TOASTing is
involved.

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

If the function was moved to the FROM clause where it would be executed as a lateral cross join instead of a target
listexpression, how would this affect the cost-based positioning of the Gather?
 

On 9/23/19, 8:59 AM, "Robert Haas" <robertmhaas@gmail.com> wrote:

    On Sun, Sep 22, 2019 at 7:47 AM Darafei "Komяpa" Praliaskouski
    <me@komzpa.net> wrote:
    > A heuristic I believe should help my case (and I hardly imagine how it can break others) is that in presence of
Gather,all the function calls that are parallel safe should be pushed into it.
 
    
    The cost of pushing data through the Sort is not necessarily
    insignificant.  Your functions are (IIUC) extremely expensive, so it's
    worth going to any length to reduce the time spent evaluating them.
    However, if someone has ||(text,text) in the tlist, that might be the
    wrong approach, because it's not saving much to compute that earlier
    and it might make the sort a lot wider, especially if de-TOASTing is
    involved.
    
    -- 
    Robert Haas
    EnterpriseDB: http://www.enterprisedb.com
    The Enterprise PostgreSQL Company
    
    
    

On Mon, Sep 23, 2019 at 9:20 PM Finnerty, Jim <jfinnert@amazon.com> wrote:
> If the function was moved to the FROM clause where it would be executed as a lateral cross join instead of a target
listexpression, how would this affect the cost-based positioning of the Gather?
 

I think you'd end up turning what is now a Seq Scan into a Nested Loop
with a Seq Scan on one side. I think the computation of the target
list would be done by the Function Scan or Result node on the other
side of the Nested Loop, and couldn't move anywhere else. The planner
would consider putting the Gather either on top of the Nested Loop or
on top of the Seq Scan, and the former would probably win. So I think
this would give the desired behavior, but I haven't thought about it
very hard.

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