Horton

Re: Removing unneeded self joins

Thomas Munro <thomas.munro@enterprisedb.com>

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From: Thomas Munro <thomas.munro@enterprisedb.com>
To: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-05-16T21:41:25Z
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 Thu, May 17, 2018 at 3:43 AM, Alexander Kuzmenkov
<a.kuzmenkov@postgrespro.ru> wrote:
> There is a join optimization we don't do -- removing inner join of a table
> with itself on a unique column. Such joins are generated by various ORMs, so
> from time to time our customers ask us to look into this. Most recently, it
> was discussed on the list in relation to an article comparing the
> optimizations that some DBMS make [1].
>
> ...
>
> I'd be glad to hear your thoughts on this.

+1

Some thoughts:

There might be some interesting corner cases involving self-joins in
UPDATE/DELETE statements, and also FOR UPDATE etc.  Those can result
in some surprising results in a self-join (one side is subject to EPQ
and the other isn't) which I think might be changed by your patch
(though I didn't try it or read the patch very closely).

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.

-- 
Thomas Munro
http://www.enterprisedb.com