Thread: [PERFORM] Query plan for views and WHERE clauses, Luke is not using the index
Hi all,
I have come across a unexpected behavior.http://paste.debian.net/982003/
WHERE "progression_geometry" && st_makeenvelope(1728327.03249295568093657,8240789.26074041239917278,1728608.10987572139129043,8240958.16933418624103069,3949)
See the query plan:
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:
The query takes about 1 second instead of less than 100ms.
Do you have any hint on this kind of issue ?
Thanks in advance
Regards,
Michaël
Re: [PERFORM] Query plan for views and WHERE clauses, Luke is not using the index
From
kimaidou
Date:
Hi all
I also tried to change the values of join_collapse_limit and rom_collapse_limit to higher values than default: 12, 50 or even 100, with no improvement on the query plan.2017-08-18 18:46 GMT+02:00 kimaidou <kimaidou@gmail.com>:
the query plan is "as expected", as it is using the spatial index (and others too). This query gets 100 lines from a "main" table containing 20000 lines (and child tables having more). It is pretty fast and "low cost"If I use the raw SQL defining the view, and add a WHERE clause like:We have indexes on some fields ( foreign keys, and a GIST index for the PostGIS geometry field)Basically, we have this view with some LEFT JOIN :You can see full detail on an issue on the QGEP project in Github :Hi all,I have come across a unexpected behavior.
http://paste.debian.net/982003/
WHERE "progression_geometry" && st_makeenvelope(1728327.03249295568093657,8240789. 26074041239917278,1728608. 10987572139129043,8240958. 16933418624103069,3949) See the query plan: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:The query takes about 1 second instead of less than 100ms.
Do you have any hint on this kind of issue ?
Thanks in advance
Regards,
Michaël
Re: [PERFORM] Query plan for views and WHERE clauses, Luke is notusing the index
From
David Rowley
Date:
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
Re: [PERFORM] Query plan for views and WHERE clauses, Luke is notusing the index
From
kimaidou
Date:
Thanks a lot for your detailed explanation. I will try ASAP with no DISTINCT ( we are quite sure it is not needed anyway ), and report back here.
Michaël2017-08-21 23:52 GMT+02:00 David Rowley <david.rowley@2ndquadrant.com>:
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