Re: BUG #19357: PostgreSQL generates a custom plan that performsworse than the generic plan for a certain query. - Mailing list pgsql-bugs

Hi Greg Sabino Mullane,

Thanks for your work.

This is already the test case that I can simplify as much as possbile. I also generate the corresponding query plan in TEXT format.

In this test case, the prepared statement (with a generic plan) is much more efficitive than the normal SELECT (with a custom plan).

```

SET plan_cache_mode = force_generic_plan;

CREATE TABLE t0(c0 smallint) USING heap WITH (parallel_workers=852);

CREATE TABLE t2(LIKE t0);

CREATE TABLE t5(LIKE t0);

INSERT INTO t5(c0) VALUES(1::INT8);

INSERT INTO t0(c0) VALUES(1::int8);

CREATE INDEX i0 ON t5(c0 NULLS FIRST);

EXPLAIN (ANALYZE, FORMAT TEXT) SELECT DISTINCT t5.c0 FROM t5, t2, t0*, (SELECT ALL t2.c0 as c0 FROM t2 WHERE ((CAST(((''::text)||('[142654042,1443301405)'::int4range)) AS BOOLEAN))) LIMIT (7461843809418659830)::int8) AS subq WHERE ((t5.c0)::VARCHAR SIMILAR TO ''::text);

PREPARE prepare_query (text, int4range, int8, text) AS SELECT DISTINCT t5.c0 FROM t5, t2*, t0*, (SELECT t2.c0 as c0 FROM t2 WHERE ((CAST((($1)||($2)) AS BOOLEAN))) LIMIT $3) AS subq WHERE ((t5.c0)::VARCHAR SIMILAR TO $4);

EXPLAIN (ANALYZE, FORMAT TEXT) EXECUTE prepare_query('', '[142654042,1443301405)'::int4range, 7461843809418659830, '');

```

This is the outputs:

```

                                                              QUERY PLAN                                                               

---------------------------------------------------------------------------------------------------------------------------------------

 Unique  (cost=522496.96..303238953.76 rows=1 width=2) (actual time=65.267..65.311 rows=0 loops=1)

   ->  Nested Loop  (cost=522496.96..252929833.76 rows=20123648000 width=2) (actual time=65.267..65.310 rows=0 loops=1)

         ->  Gather Merge  (cost=522496.94..1384162.54 rows=7398400 width=2) (actual time=65.266..65.309 rows=0 loops=1)

               Workers Planned: 2

               Workers Launched: 2

               ->  Sort  (cost=521496.92..529203.59 rows=3082667 width=2) (actual time=21.743..21.744 rows=0 loops=3)

                     Sort Key: t5.c0

                     Sort Method: quicksort  Memory: 25kB

                     Worker 0:  Sort Method: quicksort  Memory: 25kB

                     Worker 1:  Sort Method: quicksort  Memory: 25kB

                     ->  Nested Loop  (cost=0.00..104956.96 rows=3082667 width=2) (actual time=21.699..21.700 rows=0 loops=3)

                           ->  Parallel Seq Scan on t0  (cost=0.00..21.33 rows=1133 width=0) (actual time=0.002..0.002 rows=0 loops=3)

                           ->  Nested Loop  (cost=0.00..65.42 rows=2720 width=2) (actual time=65.088..65.089 rows=0 loops=1)

                                 ->  Seq Scan on t5  (cost=0.00..1.02 rows=1 width=2) (actual time=65.088..65.088 rows=0 loops=1)

                                       Filter: (((c0)::character varying)::text ~ '^(?:)$'::text)

                                       Rows Removed by Filter: 1

                                 ->  Seq Scan on t2  (cost=0.00..37.20 rows=2720 width=0) (never executed)

         ->  Materialize  (cost=0.01..78.01 rows=2720 width=0) (never executed)

               ->  Subquery Scan on subq  (cost=0.01..64.41 rows=2720 width=0) (never executed)

                     ->  Limit  (cost=0.01..37.21 rows=2720 width=2) (never executed)

                           ->  Result  (cost=0.01..37.21 rows=2720 width=2) (never executed)

                                 One-Time Filter: ((''::text || ('[142654042,1443301405)'::int4range)::text))::boolean

                                 ->  Seq Scan on t2 t2_1  (cost=0.01..37.21 rows=2720 width=0) (never executed)

 Planning Time: 0.195 ms

 JIT:

   Functions: 21

   Options: Inlining true, Optimization true, Expressions true, Deforming true

   Timing: Generation 0.636 ms (Deform 0.114 ms), Inlining 34.664 ms, Optimization 18.423 ms, Emission 11.975 ms, Total 65.698 ms

 Execution Time: 74.751 ms

(29 rows)

                                                          QUERY PLAN                                                          

------------------------------------------------------------------------------------------------------------------------------

 HashAggregate  (cost=30194812.07..30194812.08 rows=1 width=2) (actual time=0.008..0.009 rows=0 loops=1)

   Group Key: t5.c0

   Batches: 1  Memory Usage: 24kB

   ->  Nested Loop  (cost=0.01..25163900.07 rows=2012364800 width=2) (actual time=0.007..0.008 rows=0 loops=1)

         ->  Nested Loop  (cost=0.01..9296.07 rows=739840 width=2) (actual time=0.007..0.007 rows=0 loops=1)

               ->  Seq Scan on t2  (cost=0.00..37.20 rows=2720 width=0) (actual time=0.007..0.007 rows=0 loops=1)

               ->  Materialize  (cost=0.01..11.55 rows=272 width=2) (never executed)

                     ->  Nested Loop  (cost=0.01..10.19 rows=272 width=2) (never executed)

                           ->  Seq Scan on t5  (cost=0.00..1.02 rows=1 width=2) (never executed)

                                 Filter: (((c0)::character varying)::text ~ similar_to_escape($4))

                           ->  Limit  (cost=0.01..3.73 rows=272 width=2) (never executed)

                                 ->  Result  (cost=0.01..37.21 rows=2720 width=2) (never executed)

                                       One-Time Filter: (($1 || ($2)::text))::boolean

                                       ->  Seq Scan on t2 t2_1  (cost=0.01..37.21 rows=2720 width=0) (never executed)

         ->  Materialize  (cost=0.00..50.80 rows=2720 width=0) (never executed)

               ->  Seq Scan on t0  (cost=0.00..37.20 rows=2720 width=0) (never executed)

 Planning Time: 0.110 ms

 JIT:

   Functions: 15

   Options: Inlining true, Optimization true, Expressions true, Deforming true

   Timing: Generation 0.208 ms (Deform 0.053 ms), Inlining 0.000 ms, Optimization 0.000 ms, Emission 0.000 ms, Total 0.208 ms

 Execution Time: 0.289 ms

(22 rows)

```

Original

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From: Greg Sabino Mullane <htamfids@gmail.com> Date: 2025-12-17 22:54 To: 798604270 <798604270@qq.com>, pgsql-bugs <pgsql-bugs@lists.postgresql.org> Subject: Re: BUG #19357: PostgreSQL generates a custom plan that performsworse than the generic plan for a certain query.

Please simplify your test query as much as possible and use "text" format in your explain results; those will improve your chances of getting a useful reply. :)