Re: Removing unneeded self joins

Andrei Lepikhov <a.lepikhov@postgrespro.ru>

From: Andrey Lepikhov <a.lepikhov@postgrespro.ru>

To: Zhihong Yu <zyu@yugabyte.com>, Ronan Dunklau <ronan.dunklau@aiven.io>

Cc: Greg Stark <stark@mit.edu>, Andres Freund <andres@anarazel.de>, Laurenz Albe <laurenz.albe@cybertec.at>, Hywel Carver <hywel@skillerwhale.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2022-08-29T05:05:29Z

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

Attachments

v37-0001-Remove-self-joins.patch (text/x-patch) patch v37-0001

On 8/29/22 04:39, Zhihong Yu wrote:
> 
> 
> On Fri, Aug 26, 2022 at 3:02 PM Zhihong Yu <zyu@yugabyte.com 
> <mailto:zyu@yugabyte.com>> wrote:
> 
>     Hi,
>     For v36-0001-Remove-self-joins.patch :
> 
>     bq removes inner join of plane table to itself
> 
>     plane table -> plain table
> 
>     For relation_has_unique_index_ext(), it seems when extra_clauses
>     is NULL, there is no need to compute `exprs`.
> 
>     Cheers
Done
> 
> 
> For remove_self_joins_recurse():
> 
> +                   if (bms_num_members(relids) > join_collapse_limit)
> +                       break;
> 
> The above line just comes out of the switch statement. This check should 
> be done again between foreach and switch.
> Otherwise the above check wouldn't achieve what you want.
> 
> Cheers
Thanks for highlighting the problem.
I guess, usage either of join_collapse_limit or from_collapse_limit 
isn't practical here.
That we really afraid here - many senseless search cycles of self-joins. 
And it may have higher limit than GUCs above. So I introduced a guc, 
called "self_join_search_limit" (so far undocumented) that is an 
explicit limit for a set of plain relations in FROM-list to search 
self-joins.

-- 
Regards
Andrey Lepikhov
Postgres Professional