Horton

Re: Removing unneeded self joins

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

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-05-16T22:37:11Z
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.

Thomas Munro <thomas.munro@enterprisedb.com> writes:
> IIUC in DB2 (the clear winner at join elimination in the article you
> mentioned), you get these sorts of things by default (optimisation
> level 5 includes it), but not if you SET CURRENT QUERY OPTIMIZATION =
> 3 as many articles recommend for OLTP work.  I think it's interesting
> that they provide that knob rather than something automatic, and
> interesting that there is one linear knob to classify your workload
> rather than N knobs for N optimisations.

There's a lot to be said for that type of approach, as opposed to trying
to drive it off some necessarily-very-inexact preliminary estimate of
query cost.  For example, the mere fact that you're joining giant tables
doesn't in itself suggest that extra efforts in query optimization will be
repaid.  (If anything, it seems more likely that the user would've avoided
silliness like useless self-joins in such a case.)

A different line of thought is that, to me, the most intellectually
defensible rationale for efforts like const-simplification and join
removal is that opportunities for those things can arise after view
expansion, even in queries where the original query text didn't seem
to contain anything extraneous.  (Robert and Andres alluded to this
upthread, but not very clearly.)  So maybe we could track how much
the query got changed during rewriting, and use that to drive the
planner's decisions about how hard to work later on.  But I'm not
very sure that this'd be superior to having a user-visible knob.

			regards, tom lane