Re: Removing unneeded self joins

Greg Stark <stark@mit.edu>

From: Greg Stark <stark@mit.edu>

To: Jaime Casanova <jcasanov@systemguards.com.ec>

Cc: Andrey Lepikhov <a.lepikhov@postgrespro.ru>, Hywel Carver <hywel@skillerwhale.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2022-03-01T02:45:19Z

Lists: pgsql-hackers

Commits

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Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination
- 717d0e8dd945 18.0 landed
Put enable_self_join_elimination into postgresql.conf.sample
- c2d329260cd8 18.0 landed
Get rid of ojrelid local variable in remove_rel_from_query()
- e167191dc146 18.0 landed
Implement Self-Join Elimination
- fc069a3a6319 18.0 cited
Revert: Remove useless self-joins
- d1d286d83c0e 17.0 landed
Replace lateral references to removed rels in subqueries
- 466979ef031a 17.0 landed
Replace relids in lateral subquery parse tree during SJE
- 489072ab7a9e 17.0 landed
Forbid SJE with result relation
- 8c441c082797 17.0 landed
Fix misuse of RelOptInfo.unique_for_rels cache by SJE
- 30b4955a4668 17.0 landed
Replace the relid in some missing fields during SJE
- a7928a57b9f0 17.0 landed
Revert 56-bit relfilenode change and follow-up commits.
- a448e49bcbe4 16.0 cited
Stabilize timetz test across DST transitions.
- 4a071afbd056 14.0 cited
Speed up finding EquivalenceClasses for a given set of rels
- 3373c7155350 13.0 cited
Fix mark-and-restore-skipping test case to not be a self-join.
- 24d08f3c0a1f 12.0 landed

I don't think the benchmarking that's needed is to check whether
pruning unnecessary joins is helpful. Obviously it's going to be hard
to measure on simple queries and small tables. But the resulting plan
is unambiguously superior and in more complex cases could extra i/o.

The benchmarking people were looking for in the past was testing the
impact of the extra planning work in cases where it doesn't end up
being applied. I'm not sure what the worst case is, perhaps a many-way
self-join where the join clauses are not suitable for pruning?