[postgis-users] diffrent execution plan on Postgres 9.5 and Postgres 11 for ST_union and performance problem Postgres 11

[Martin Davis]

Yes, that's exactly the test needed.  So it looks like ST_Union(agg) on the
11 instance is much slower than on 9.5:

11: 25335.136 ms
9.5: 4409.919 ms

The question is why...  We'll have a look and see if there's any regression
in GEOS (which is what is performing the union).

Are you able to share the dataset from the above test?  (As a file of WKT
or WKB)

On Thu, Apr 11, 2019 at 11:55 AM Lars Aksel Opsahl <Lars.Opsahl at nibio.no>
wrote:

> Hi
>
> Do mean something like this ?
>
> Here is the result from postgres 11 -
>
> EXPLAIN ANALYZE
>
> select (ST_dump(st_union(geo))).geom as geo
>
> from sde_markslag.markslag_myrikilden_temp
>
> where gid  < 10000
>
> ;
>
>                                                                     QUERY
> PLAN
>
>
> ---------------------------------------------------------------------------------------------------------------------------------------------------
>
>  Result  (cost=62634.26..62899.53 rows=1000 width=32) (actual
> time=25249.956..25331.325 rows=9205 loops=1)
>
>    ->  ProjectSet  (cost=62634.26..62639.53 rows=1000 width=32) (actual
> time=25249.953..25330.483 rows=9205 loops=1)
>
>          ->  Aggregate  (cost=62634.26..62634.28 rows=1 width=32) (actual
> time=25244.718..25244.719 rows=1 loops=1)
>
>                ->  Seq Scan on markslag_myrikilden_temp
> (cost=0.00..62609.51 rows=9900 width=1593) (actual time=0.008..173.033
> rows=9999 loops=1)
>
>                      Filter: (gid < 10000)
>
>                      Rows Removed by Filter: 557242
>
>  Planning Time: 0.064 ms
>
>  Execution Time: 25335.136 ms
>
> (8 rows)
>
>
> Here is the same postgres 9.5 -
>
> EXPLAIN ANALYZE
>
> [more] - > select (ST_dump(st_union(geo))).geom as geo
>
> [more] - > from sde_markslag.markslag_myrikilden_temp
>
> [more] - > where gid  < 10000
>
> [more] - > ;
>
>                                                                QUERY PLAN
>
>
>
> ----------------------------------------------------------------------------------------------------------------------------------------
>
>  Aggregate  (cost=170462.27..170467.28 rows=1000 width=1604) (actual
> time=4311.911..4409.250 rows=9205 loops=1)
>
>    ->  Seq Scan on markslag_myrikilden_temp  (cost=0.00..170437.51
> rows=9903 width=1604) (actual time=0.054..629.045 rows=9999 loops=1)
>
>          Filter: (gid < 10000)
>
>          Rows Removed by Filter: 557242
>
>  Planning time: 0.045 ms
>
>  Execution time: 4409.919 ms
>
> (6 rows)
>
>
> Thanks
>
> Lars
>
> ------------------------------
> *From:* postgis-users <postgis-users-bounces at lists.osgeo.org> on behalf
> of Martin Davis <mtnclimb at gmail.com>
> *Sent:* Thursday, April 11, 2019 8:07 PM
> *To:* PostGIS Users Discussion
> *Subject:* Re: [postgis-users] diffrent execution plan on Postgres 9.5
> and Postgres 11 for ST_union and performance problem Postgres 11
>
>
>
> On Thu, Apr 11, 2019 at 3:27 AM Lars Aksel Opsahl <Lars.Opsahl at nibio.no>
> wrote:
>
> Can it be problem related ST_Union and aggregate ?
>
>
> Can you test the performance of aggregate ST_Union on it's own by dropping
> the GROUP BY and instead using a WHERE clause to union a small-ish subset
> of the geometries?  (But large enough to give a measurable query time.)
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