Re: Removing unneeded self joins

Tom Lane <tgl@sss.pgh.pa.us>

From: Tom Lane <tgl@sss.pgh.pa.us>

To: David Rowley <david.rowley@2ndquadrant.com>

Cc: Simon Riggs <simon@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>

Date: 2018-05-16T22:13:51Z

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

David Rowley <david.rowley@2ndquadrant.com> writes:
> On 17 May 2018 at 08:44, Simon Riggs <simon@2ndquadrant.com> wrote:
>> What I was advocating was an approach that varies according to the
>> query cost, so we don't waste time trying to tune the heck out of OLTP
>> queries, but for larger queries we might take a more considered
>> approach.

> That's tricky. If we do this, it should be done before Path
> generation, so not much is known about the costs in those case.

Yeah.  It'd have to be a very heuristic thing that doesn't account
for much beyond the number of relations in the query, and maybe their
sizes --- although I don't think we even know the latter at the
point where join removal would be desirable.  (And note that one of
the desirable benefits of join removal is not having to find out the
sizes of removed rels ... so just swapping that around doesn't appeal.)

			regards, tom lane