pgsql: plpgsql: Don't generate parallel plans for RETURN QUERY.

From: Robert Haas <rhaas(at)postgresql(dot)org>
To: pgsql-committers(at)postgresql(dot)org
Subject: pgsql: plpgsql: Don't generate parallel plans for RETURN QUERY.
Date: 2017-03-24 16:43:54
Message-ID: E1crSJe-0002VE-Kk@gemulon.postgresql.org
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plpgsql: Don't generate parallel plans for RETURN QUERY.

Commit 7aea8e4f2daa4b39ca9d1309a0c4aadb0f7ed81b allowed a parallel
plan to be generated when for a RETURN QUERY or RETURN QUERY EXECUTE
statement in a PL/pgsql block, but that's a bad idea because plplgsql
asks the executor for 50 rows at a time. That means that we'll always
be running serially a plan that was intended for parallel execution,
which is not a good idea. Fix by not requesting a parallel plan from
the outset.

Per discussion, back-patch to 9.6. There is a slight risk that, due
to optimizer error, somebody could have a case where the parallel plan
executed serially is actually faster than the supposedly-best serial
plan, but the consensus seems to be that that's not sufficient
justification for leaving 9.6 unpatched.

Discussion: http://postgr.es/m/CA+TgmoZ_ZuH+auEeeWnmtorPsgc_SmP+XWbDsJ+cWvWBSjNwDQ@mail.gmail.com
Discussion: http://postgr.es/m/CA+TgmobXEhvHbJtWDuPZM9bVSLiTj-kShxQJ2uM5GPDze9fRYA@mail.gmail.com

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5674a258fd7e6eb496a4e91b0907077dfa7ee336

Modified Files
--------------
src/pl/plpgsql/src/pl_exec.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

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