Horton

Re: Removing unneeded self joins

Alexander Lakhin <exclusion@gmail.com>

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From: Alexander Lakhin <exclusion@gmail.com>
To: Alexander Korotkov <aekorotkov@gmail.com>, Andrei Lepikhov <a.lepikhov@postgrespro.ru>
Cc: "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-04-30T06:00:00Z
Lists: pgsql-hackers

Commits

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  1. Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination

  2. Put enable_self_join_elimination into postgresql.conf.sample

  3. Get rid of ojrelid local variable in remove_rel_from_query()

  4. Implement Self-Join Elimination

  5. Revert: Remove useless self-joins

  6. Replace lateral references to removed rels in subqueries

  7. Replace relids in lateral subquery parse tree during SJE

  8. Forbid SJE with result relation

  9. Fix misuse of RelOptInfo.unique_for_rels cache by SJE

  10. Replace the relid in some missing fields during SJE

  11. Revert 56-bit relfilenode change and follow-up commits.

  12. Stabilize timetz test across DST transitions.

  13. Speed up finding EquivalenceClasses for a given set of rels

  14. Fix mark-and-restore-skipping test case to not be a self-join.

Hello Alexander,

23.10.2023 12:47, Alexander Korotkov wrote:
> I think this patch makes substantial improvement to query planning.
> It has received plenty of reviews.  The code is currently in quite
> good shape.  I didn't manage to find the cases when this optimization
> causes significant overhead to planning time.  Even if such cases will
> be spotted there is a GUC option to disable this feature.  So, I'll
> push this if there are no objections.

I've discovered another failure, introduced by d3d55ce57.
Please try the following:
CREATE TABLE t (a int unique, b float);
SELECT * FROM t NATURAL JOIN LATERAL
  (SELECT * FROM t t2 TABLESAMPLE SYSTEM (t.b)) t2;

With asserts enabled, it triggers
TRAP: failed Assert("!bms_is_member(rti, lateral_relids)"), File: "initsplan.c", Line: 697, PID: 3074054
ExceptionalCondition at assert.c:52:13
create_lateral_join_info at initsplan.c:700:8
query_planner at planmain.c:257:2
grouping_planner at planner.c:1523:17
subquery_planner at planner.c:1098:2
standard_planner at planner.c:415:9
planner at planner.c:282:12
pg_plan_query at postgres.c:904:9
pg_plan_queries at postgres.c:996:11
exec_simple_query at postgres.c:1193:19
PostgresMain at postgres.c:4684:27

With no asserts, I get:
ERROR:  failed to construct the join relation

Please take a look at this.

Best regards,
Alexander