Re: Removing unneeded self joins

Andrei Lepikhov <lepihov@gmail.com>

From: Andrei Lepikhov <lepihov@gmail.com>

To: Alexander Korotkov <aekorotkov@gmail.com>

Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tom Lane <tgl@sss.pgh.pa.us>, 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: 2025-01-31T04:57:46Z

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

Attachments

v10-0001-Remove-useless-self-joins.patch (text/x-patch) patch v10-0001

On 1/10/25 10:13, Alexander Korotkov wrote:
> I've got an off-list report from Alexander Lakhin.  The failing query
> is added to the regression tests in the revised patch.  The query was
> failing with an error "negative bitmapset member not allowed" issued
> in adjust_relid_set().  In order to fix that I've to teach
> adjust_relid_set() about negative newrelid, it became even more
> similar to replace_relid().  Notable this error happens in
> remove_leftjoinrel_from_query(). This seems to be consequence that we
> made self-join removal and left-joins removal use common
> infrastructure.
I have analysed your change. It pertains to the situation where PHV 
references both the outer join and its inner join in phrels but actually 
utilises only the outer side of the join.

I agree with your proposed fix and have just rewritten the query to make 
it simpler and more consistent with the regression tests (see new 
version of the patch in attachment).

Additionally, I noticed that the remove_useless_joins code uses the term 
"join removal" in the join.sql tests. I wonder if it might be more 
appropriate to revert the name from "Self Join Elimination" back to 
"Self Join Removal."

-- 
regards, Andrei Lepikhov