Re: Removing unneeded self joins

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

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

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

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

Date: 2018-05-17T01:33:37Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

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

On 17 May 2018 at 10:55, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> David Rowley <david.rowley@2ndquadrant.com> writes:
>> 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.
>
> (But wait a second ... we could improve this without quite that much work:
> instead of doing estimate_rel_size immediately during get_relation_info,
> couldn't it be left until the set_base_rel_sizes pass?  Since
> RelationGetNumberOfBlocks involves kernel calls, skipping it for removed
> rels seems worth doing.)

I did mean just obtaining the sizes, not delaying building the
RelOptInfo. I see nothing that needs RelOptInfo->pages before
set_base_rel_size apart from the code which I mentioned about moving a
couple of days ago in [1].

[1] https://www.postgresql.org/message-id/CAKJS1f_eUz0_h5_vU1rqE7wuxMcoENcWK2FTODz0pOyxp3_Uig%40mail.gmail.com

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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