Re: Removing unneeded self joins

Andrei Lepikhov <a.lepikhov@postgrespro.ru>

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

To: Greg Stark <stark@mit.edu>, Ronan Dunklau <ronan.dunklau@aiven.io>

Cc: Laurenz Albe <laurenz.albe@cybertec.at>, Hywel Carver <hywel@skillerwhale.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Konstantin Knizhnik <knizhnik@garret.ru>

Date: 2022-03-04T10:47:47Z

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

v30-0001-Remove-self-joins.patch (text/plain) patch v30-0001
ec_usage_demo.sql (application/sql)

On 1/3/2022 03:03, Greg Stark wrote:
> On Thu, 1 Jul 2021 at 02:38, Ronan Dunklau <ronan.dunklau@aiven.io> wrote:
>>
>> Well in some cases they can't, when the query is not emitting redundant
>> predicates by itself but they are added by something else like a view or a RLS
>> policy.
>> Maybe it would be worth it to allow spending a bit more time planning for
>> those cases ?
> 
> Yeah, I'm generally in favour of doing more work in the optimizer to
> save query authors work writing queries.
> 
> My question is whether it handles cases like:
> 
> select b.x,c.y
>    from t
>     join t2 as b on (b.id = t.id)
>        join t2 as c on (c.id = t.id)
> 
> That is, if you join against the same table twice on the same qual.
> Does the EC mechanism turn this into a qual on b.id = c.id and then
> turn this into a self-join that can be removed?
Yes, the self-join removal machinery uses EC mechanism as usual to get 
all join clauses. So, this case works (See demo in attachment).

Also, in new version of the patch I fixed one stupid bug: checking a 
self-join candidate expression operator - we can remove only expressions 
like F(arg1) = G(arg2).

-- 
regards,
Andrey Lepikhov
Postgres Professional