Re: Planner makes sub-optimal execution plan - Mailing list pgsql-performance

Hi! Thank you for sharing this interesting case!

On 01.09.2025 12:07, Алексей Борщёв wrote:
> EXPLAIN (ANALYZE, VERBOSE, BUFFERS, SUMMARY, SETTINGS, TIMING)
> SELECT
>      MIN(docum.dt) AS "dt__min",
>      MAX(docum.dt_real) AS "dt_real__max"
> FROM docum
> WHERE docum.dt_real >= '2025-08-14T09:44:09.033592'::timestamp;
> -- The plan I've got is:
> Result  (cost=8.38..8.39 rows=1 width=16) (actual
> time=2660.034..2660.036 rows=1 loops=1)
>    Output: (InitPlan 1).col1, (InitPlan 2).col1
>    Buffers: shared hit=9358751 read=30994 written=1
>    InitPlan 1
>      ->  Limit  (cost=0.43..7.91 rows=1 width=8) (actual
> time=2660.006..2660.007 rows=1 loops=1)
>            Output: docum.dt
>            Buffers: shared hit=9358747 read=30994 written=1
>            ->  Index Scan using docum_dt_7ee1d676 on public.docum
> (cost=0.43..420487.43 rows=56222 width=8) (actual
> time=2660.004..2660.005 rows=1 loops=1)
>                  Output: docum.dt
>                  Index Cond: (docum.dt IS NOT NULL)
>                  Filter: (docum.dt_real >= '2025-08-14
> 09:44:09.033592'::timestamp without time zone)
>                  Rows Removed by Filter: 11342966
>                  Buffers: shared hit=9358747 read=30994 written=1
>    InitPlan 2
>      ->  Limit  (cost=0.43..0.46 rows=1 width=8) (actual
> time=0.022..0.022 rows=1 loops=1)
>            Output: docum_1.dt_real
>            Buffers: shared hit=4
>            ->  Index Only Scan Backward using docum_dt_real_2b81c58c on
> public.docum docum_1  (cost=0.43..1689.22 rows=59245 width=8) (actual
> time=0.021..0.021 rows=1 loops=1)
>                  Output: docum_1.dt_real
>                  Index Cond: (docum_1.dt_real >= '2025-08-14
> 09:44:09.033592'::timestamp without time zone)
>                  Heap Fetches: 0
>                  Buffers: shared hit=4
> Settings: work_mem = '16MB', search_path = 'public, public, "$user"'
> Planning:
>    Buffers: shared hit=12
> Planning Time: 0.148 ms
> Execution Time: 2660.056 ms


After disabling MIN/MAX optimization in the grouping_planner function:
/*
          * Preprocess MIN/MAX aggregates, if any.  Note: be careful about
          * adding logic between here and the query_planner() call.  
Anything
          * that is needed in MIN/MAX-optimizable cases will have to be
          * duplicated in planagg.c.
          */
         //if (parse->hasAggs)
         //    preprocess_minmax_aggregates(root);

I got a better query plan, but I’m still investigating what went wrong.


Also, creating a partial index helped because of the use of indexonlyscan:

CREATE INDEX CONCURRENTLY docum_dt_recent
   ON docum (dt)
   WHERE dt_real >= timestamp '2025-08-01 00:00:00';

postgres=# EXPLAIN (ANALYZE, VERBOSE, BUFFERS, SUMMARY, SETTINGS, TIMING)
SELECT
     MIN(docum.dt) AS "dt__min",
     MAX(docum.dt_real) AS "dt_real__max"
FROM docum
WHERE docum.dt_real >= '2025-08-14T09:44:09.033592'::timestamp;
QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Result  (cost=1.11..1.12 rows=1 width=16) (actual 
time=206.206..206.207 rows=1.00 loops=1)
    Output: (InitPlan 1).col1, (InitPlan 2).col1
    Buffers: shared hit=212507
    InitPlan 1
      ->  Limit  (cost=0.42..0.64 rows=1 width=8) (actual 
time=206.154..206.155 rows=1.00 loops=1)
            Output: docum.dt
            Buffers: shared hit=212503
            ->  Index Scan using docum_dt_recent on public.docum  
(cost=0.42..12391.88 rows=55710 width=8) (actual time=206.150..206.150 
rows=1.00 loops=1)
                  Output: docum.dt
                  Index Cond: (docum.dt IS NOT NULL)
                  Filter: (docum.dt_real >= '2025-08-14 
09:44:09.033592'::timestamp without time zone)
                  Rows Removed by Filter: 256799
                  Index Searches: 1
                  Buffers: shared hit=212503
    InitPlan 2
      ->  Limit  (cost=0.43..0.46 rows=1 width=8) (actual 
time=0.042..0.042 rows=1.00 loops=1)
            Output: docum_1.dt_real
            Buffers: shared hit=4
            ->  Index Only Scan Backward using docum_dt_real on 
public.docum docum_1  (cost=0.43..1671.62 rows=58696 width=8) (actual 
time=0.041..0.041 rows=1.00 loops=1)
                  Output: docum_1.dt_real
                  Index Cond: (docum_1.dt_real >= '2025-08-14 
09:44:09.033592'::timestamp without time zone)
                  Heap Fetches: 0
                  Index Searches: 1
                  Buffers: shared hit=4
  Planning:
    Buffers: shared hit=30 read=10
  Planning Time: 2.512 ms
  Execution Time: 206.290 ms
(28 rows)

--

Regards,
Alena Rybakina
Postgres Professional