Horton

Re: Removing unneeded self joins

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <david.rowley@2ndquadrant.com>, Simon Riggs <simon@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-05-16T23:00:53Z
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.

On 2018-05-16 18:55:41 -0400, Tom Lane wrote:
> David Rowley <david.rowley@2ndquadrant.com> writes:
> > On 17 May 2018 at 10:13, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> 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.)
> 
> > There's probably some argument for delaying obtaining the relation
> > size until after join removal and probably partition pruning too, but
> > it's currently done well before that in build_simple_rel, where the
> > RelOptInfo is built.
> 
> Yeah, but that's something we ought to fix someday; IMO it's an artifact
> of having wedged in remove_useless_joins without doing the extensive
> refactoring that'd be needed to do it at a more desirable time.  I don't
> want to build user-visible behavior that's dependent on doing that wrong.

My patch that introduced a radix tree buffer mapping also keeps an
accurate relation size in memory, making it far cheaper to use. While I
depriorized the patchset for the moment (I'll post what I'm working on
first soon), that should address some of the cost till then.

Wonder if we shouldn't just cache an estimated relation size in the
relcache entry till then. For planning purposes we don't need to be
accurate, and usually activity that drastically expands relation size
will trigger relcache activity before long. Currently there's plenty
workloads where the lseeks(SEEK_END) show up pretty prominently.

Greetings,

Andres Freund