Horton

Re: Removing unneeded self joins

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>, Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-05-16T20:44:27Z
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 16 May 2018 at 15:10, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
>> What I would add is that I've seen cases where the extra joins do NOT
>> hurt performance, so the extra CPU used to remove the join hurts more
>> than the benefit of removing it. Yes, we tried it.
>
> Interesting.  The concern I had was more about the cost imposed on every
> query to detect self-joins and try to prove them useless, even in queries
> where no benefit ensues.  It's possible that we can get that down to the
> point where it's negligible; but this says that even the successful-proof
> case has to be very cheap.

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.

For advanced optimizations that are costly to check for, skip the
check if we are already below a cost threshold. The threshold would be
a heuristic that varies according to the cost of the check.

I realise that in this case we wouldn't know the full query cost until
we've done join planning, so we would need some lower bound estimate
to check whether its worth trying to remove joins.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services