Horton

Re: Removing unneeded self joins

Andrei Lepikhov <a.lepikhov@postgrespro.ru>

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From: "Andrey V. Lepikhov" <a.lepikhov@postgrespro.ru>
To: Heikki Linnakangas <hlinnaka@iki.fi>, David Rowley <dgrowleyml@gmail.com>
Cc: David Steele <david@pgmasters.net>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-11-30T05:50:57Z
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.

Attachments

On 11/29/20 10:10 PM, Heikki Linnakangas wrote:
> On 28/11/2020 19:21, Andrey Lepikhov wrote:
>> On 27.11.2020 21:49, Heikki Linnakangas wrote:
>> CREATE TABLE a(x int, y int);
>> CREATE UNIQUE INDEX ON a(x);
>> SELECT a1.* FROM a a1, a a2 WHERE a1.x = a2.x;  -- self-join
>> CREATE UNIQUE INDEX ON a(y);
>> SELECT a1.* FROM a a1, a a2 WHERE a1.x = a2.y;  -- self-join too
> 
> The latter join is not "useless". The patch is returning incorrect 
> result for that query:
> 
>> postgres=# insert into a values (1, 2);
>> INSERT 0 1
>> postgres=# insert into a values (2, 1);
>> INSERT 0 1
>> postgres=# SELECT a1.* FROM a a1, a a2 WHERE a1.x = a2.y; -- WRONG RESULT
>>  x | y ---+---
>> (0 rows)
>>
>> postgres=# set enable_self_join_removal=off;
>> SET
>> postgres=# SELECT a1.* FROM a a1, a a2 WHERE a1.x = a2.y; -- CORRECT 
>> RESULT
>>  x | y ---+---
>>  1 | 2
>>  2 | 1
>> (2 rows)

Thanks, it is my fault. I tried to extend this patch with foreign key 
references and made a mistake.
Currently I rollback this new option (see patch in attachment), but will 
be working for a while to simplify this patch.

-- 
regards,
Andrey Lepikhov
Postgres Professional