Re: Removing unneeded self joins

Andrei Lepikhov <lepihov@gmail.com>

From: Andrei Lepikhov <lepihov@gmail.com>

To: Alexander Korotkov <aekorotkov@gmail.com>

Cc: Tom Lane <tgl@sss.pgh.pa.us>, Alena Rybakina <a.rybakina@postgrespro.ru>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Robert Haas <robertmhaas@gmail.com>, Richard Guo <guofenglinux@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, Dean Rasheed <dean.a.rasheed@gmail.com>

Date: 2025-03-07T06:03:53Z

Lists: pgsql-hackers

Commits

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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 26/2/2025 13:14, Alexander Korotkov wrote:
> On Mon, Feb 24, 2025 at 2:22 PM Andrei Lepikhov <lepihov@gmail.com> wrote:
>> On 24/2/2025 11:57, Alexander Korotkov wrote:
>>> Could you, please, elaborate more on what you mean by "new technique
>>> of query tree reduction"?
>> I mean any transformations and optimisations that reduce search space
>> for optimisation. Right now, I see the features reduce_unique_semijoins,
>> remove_useless_joins, and remove_useless_self_joins.
>> In practice, I see at least a join on a foreign key, where some cases
>> potentially allow the removal of the JOIN operator.
> 
> Do you mean some generic facility, which generalizes all the
> transformations you mentioned?  If so, it would be cool.  But how
> could it look like?
I think we may realise what it may look like by attempting to implement 
more RelOptInfo-removal features. Right now, I have only vague ideas on 
that subject. Just for reference, there were other discussions on join 
removal [1 - 5] and some blog posts explaining Oracle techniques in this 
area - see, for example, [6].

[1] inner join removal
https://www.postgresql.org/message-id/flat/AANLkTinS_MlZ2F3Siwgcje--qf5nTJTpuHFnZdcA45bU%40mail.gmail.com
[2] Patch to support SEMI and ANTI join removal
https://www.postgresql.org/message-id/flat/CAApHDvpCBEfuc5tD%3DvniepAv0pU5m%3Dq%3DfOQZcOdMHeei7OQPgQ%40mail.gmail.com
[3] Removing INNER JOINs
https://www.postgresql.org/message-id/flat/CAApHDvocUEYdt1uT+DLDPs2xEu=v3qJGT6HeXKonQM4rY_OsSA@mail.gmail.com#CAApHDvocUEYdt1uT+DLDPs2xEu=v3qJGT6HeXKonQM4rY_OsSA@mail.gmail.com
[4] WIP Join Removal
https://www.postgresql.org/message-id/flat/1220176372.4371.118.camel%40ebony.2ndQuadrant
[5] Join Removal/ Vertical Partitioning
https://www.postgresql.org/message-id/flat/1214477827.3845.87.camel%40ebony.site
[6] Oracle Join Elimination
https://oracle-base.com/articles/misc/join-elimination

-- 
regards, Andrei Lepikhov