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: 2025-02-24T12:22:35Z

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 24/2/2025 11:57, Alexander Korotkov wrote:
> Hi, Andrei!
> 
> Thank you for your feedback.
> 
> On Mon, Feb 24, 2025 at 12:12 AM Andrei Lepikhov <lepihov@gmail.com> wrote:
>> On 23/2/2025 22:15, Alexander Korotkov wrote:
>>> There is my attempt to implement this approach.  Getting rid of local
>>> variable (and computation of the same value other way) required to
>>> change arguments of remove_rel_from_eclass() as well.  I'm going to
>>> further polish this tomorrow.
>> I passed through the patch. It works correctly.
>>
>> Multiple ifs in a single routine is not ideal.
> 
> I would say there is a brachcing anyway.  Even if it is inside
> adjust_relid_set().  Patch makes it more explicit.  Ideal or not, I
> don't think it gets worse.
> 
>> I have thought about it
>> already, and it seems the remove_rel_from_query needs refactoring: when
>> I first reused it for self-join removal, we didn't have the 'ojrelid'
>> machinery, and it was implemented smoothly.
>> Right now, this code contains multiple places where we need to remove
>> the outer join relid and separating the removal code for the baserel and
>> outer join may simplify the logic and make it safer.
>> But the way to do it is not apparent now. May be if we implement a new
>> technique of query tree reduction, the approach will become more evident.
> 
> 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.

-- 
regards, Andrei Lepikhov