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

David G. Johnston <david.g.johnston@gmail.com>

From: "David G. Johnston" <david.g.johnston@gmail.com>

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

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

Date: 2018-08-13T20:25:09Z

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

On Monday, August 13, 2018, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> 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.
>
>
Could the planner stick a materialize node there that feeds the same set of
originally selected records to any parallel executors that end up pulling
from it?

David J.