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From: David Rowley <david.rowley@2ndquadrant.com>
Date: Tue, 22 Aug 2017 09:52:17 +1200
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 <CAKJS1f9qXZGBWu_Mx=aLEs8phH3ttc6RgMEo66FpHcb6Mf+nTQ@mail.gmail.com>
Subject: Re: Query plan for views and WHERE clauses, Luke is not
 using the index
To: kimaidou <kimaidou@gmail.com>
Cc: postgres performance list <pgsql-performance@postgresql.org>
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On 19 August 2017 at 04:46, kimaidou <kimaidou@gmail.com> wrote:
> When we call the WHERE on the view:
>
> EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS)
> SELECT *
> FROM "qgep"."vw_qgep_reach"
> WHERE "progression_geometry" &&
> st_makeenvelope(1728327.03249295568093657,8240789.26074041239917278,1728608.10987572139129043,8240958.16933418624103069,3949)
>
>
> The query plan is "wrong", as PostgreSQL seems to consider it should do a
> seq scan on the tables, and only afterwards filter with the WHERE:
> https://explain.depesz.com/s/wXV
>
> The query takes about 1 second instead of less than 100ms.
>
> Do you have any hint on this kind of issue ?

This is by design due to the DISTINCT ON() clause. Only quals which
filter columns which are in the DISTINCT ON can be safely pushed down.

Consider the following, where I've manually pushed the WHERE clause.

postgres=# create table tt (a int, b int);
CREATE TABLE
postgres=# create index on tt (a);
CREATE INDEX
postgres=# insert into tt values(1,1),(1,2),(2,1),(2,2);
INSERT 0 4
postgres=# select * from (select distinct on (a) a,b from tt order by
a,b) tt where b = 2;
 a | b
---+---
(0 rows)


postgres=# select * from (select distinct on (a) a,b from tt where b =
2 order by a,b) tt;
 a | b
---+---
 1 | 2
 2 | 2
(2 rows)

Note the results are not the same.

If I'd done WHERE a = 2, then the planner would have pushed the qual
down into the subquery.

More reading in check_output_expressions() in allpaths.c:

/* If subquery uses DISTINCT ON, check point 3 */
if (subquery->hasDistinctOn &&
!targetIsInSortList(tle, InvalidOid, subquery->distinctClause))
{
/* non-DISTINCT column, so mark it unsafe */
safetyInfo->unsafeColumns[tle->resno] = true;
continue;
}

The comment for point 3 reads:

 * 3. If the subquery uses DISTINCT ON, we must not push down any quals that
 * refer to non-DISTINCT output columns, because that could change the set
 * of rows returned.  (This condition is vacuous for DISTINCT, because then
 * there are no non-DISTINCT output columns, so we needn't check.  Note that
 * subquery_is_pushdown_safe already reported that we can't use volatile
 * quals if there's DISTINCT or DISTINCT ON.)


-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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