Horton

Re: Removing unneeded self joins

Andrei Lepikhov <a.lepikhov@postgrespro.ru>

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From: Andrei Lepikhov <a.lepikhov@postgrespro.ru>
To: "a.rybakina" <a.rybakina@postgrespro.ru>, Alexander Korotkov <aekorotkov@gmail.com>
Cc: "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-10-13T09:03:50Z
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 →

  1. Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination

  2. Put enable_self_join_elimination into postgresql.conf.sample

  3. Get rid of ojrelid local variable in remove_rel_from_query()

  4. Implement Self-Join Elimination

  5. Revert: Remove useless self-joins

  6. Replace lateral references to removed rels in subqueries

  7. Replace relids in lateral subquery parse tree during SJE

  8. Forbid SJE with result relation

  9. Fix misuse of RelOptInfo.unique_for_rels cache by SJE

  10. Replace the relid in some missing fields during SJE

  11. Revert 56-bit relfilenode change and follow-up commits.

  12. Stabilize timetz test across DST transitions.

  13. Speed up finding EquivalenceClasses for a given set of rels

  14. Fix mark-and-restore-skipping test case to not be a self-join.

On 13/10/2023 15:56, a.rybakina wrote:
> 
>>> Also I've incorporated improvements from Alena Rybakina except one for
>>> skipping SJ removal when no SJ quals is found.  It's not yet clear for
>>> me if this check fix some cases. But at least optimization got skipped
>>> in some useful cases (as you can see in regression tests).
>>
>> Agree. I wouldn't say I like it too. But also, I suggest skipping some 
>> unnecessary assertions proposed in that patch:
>> Assert(toKeep->relid != -1); - quite strange. Why -1? Why not all the 
>> negative numbers, at least?
>> Assert(is_opclause(orinfo->clause)); - above we skip clauses with 
>> rinfo->mergeopfamilies == NIL. Each mergejoinable clause is already 
>> checked as is_opclause.
>> All these changes (see in the attachment) are optional.
>>
> I don't mind about asserts, maybe I misunderstood something in the patch.
> 
> About skipping SJ removal when no SJ quals is found, I assume it is 
> about it:
> 
> split_selfjoin_quals(root, restrictlist, &selfjoinquals,
>                                    &otherjoinquals, inner->relid, 
> outer->relid);
> 
> +            if (list_length(selfjoinquals) == 0)
> +             {
> +                 /*
> +                  * XXX:
> +                  * we would detect self-join without quals like 'x==x' 
> if we had
> +                  * an foreign key constraint on some of other quals 
> and this join
> +                  * haven't any columns from the outer in the target list.
> +                  * But it is still complex task.
> +                  */
> +                 continue;
> +             }
> 
> as far as I remember, this is the place where it is checked that the SJ 
> list is empty and it is logical, in my opinion, that no transformations 
> should be performed if no elements are found for them.
You forget we have "Degenerate" case, as Alexander mentioned above. What 
if you have something like that:
SELECT ... FROM A a1, A a2 WHERE a1.id=1 AND a2.id=1;
In this case, uniqueness can be achieved by the baserestrictinfo 
"A.id=1", if we have an unique index on this column.

-- 
regards,
Andrey Lepikhov
Postgres Professional