Re: BUG #15324: Non-deterministic behaviour from parallelised sub-query

Tom Lane <tgl@sss.pgh.pa.us>

From: Tom Lane <tgl@sss.pgh.pa.us>

To: Pavel Stehule <pavel.stehule@gmail.com>

Cc: Marko Tiikkaja <marko@joh.to>, Andres Freund <andres@anarazel.de>, andy@prestigedigital.com, pgsql-bugs@lists.postgresql.org

Date: 2018-08-13T18:04:47Z

Lists: pgsql-bugs

Commits

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Don't allow LIMIT/OFFSET clause within sub-selects to be pushed to workers.
- 568b4e1fdeb3 9.6.11 landed
- 1ceb103e7d1e 10.6 landed
- 830d7565902f 11.0 landed
- 75f9c4ca5a80 12.0 landed
Back-patch "Fix parallel hash join path search."
- 271b678436ce 9.6.11 landed
Prohibit pushing subqueries containing window function calculation to
- f658235a448a 9.6.11 landed
- bf61873ae3b6 10.6 landed
- 14e9b2a752ef 12.0 landed
- 2ce253cf57b1 11.0 landed
Fix parallel hash join path search.
- 655393a022bd 10.0 cited

Pavel Stehule <pavel.stehule@gmail.com> writes:
> 2018-08-13 19:26 GMT+02:00 Tom Lane <tgl@sss.pgh.pa.us>:
>> Likely, we need to treat the presence of a LIMIT/OFFSET in a sub-select
>> as making it parallel-unsafe, for exactly the reason that that makes
>> its results non-deterministic.

> Isn't it default behave of LIMIT/OFFSET without ORDER BY clause?

In principle, the planner could prove in some cases that the results
were deterministic even with LIMIT/OFFSET.  BuT I doubt it's worth
the trouble.  I certainly wouldn't advocate for such logic to be
part of a back-patched bug fix.

			regards, tom lane