From: | Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> |
---|---|
To: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
Cc: | Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>, Gilles Darold <gilles(dot)darold(at)dalibo(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: [HACKERS] proposal: schema variables |
Date: | 2018-09-04 07:21:07 |
Message-ID: | CAEZATCVy77hhgTZ1Yrvbp6GbsUuWY0Bw8CLbv_Jr2mAF_bOkBA@mail.gmail.com |
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Lists: | pgsql-hackers |
AFAICS this patch does nothing to consider parallel safety -- that is,
as things stand, a variable is allowed in a query that may be
parallelised, but its value is not copied to workers, leading to
incorrect results. For example:
create table foo(a int);
insert into foo select * from generate_series(1,1000000);
create variable zero int;
let zero = 0;
explain (costs off) select count(*) from foo where a%10 = zero;
QUERY PLAN
-----------------------------------------------
Finalize Aggregate
-> Gather
Workers Planned: 2
-> Partial Aggregate
-> Parallel Seq Scan on foo
Filter: ((a % 10) = zero)
(6 rows)
select count(*) from foo where a%10 = zero;
count
-------
38037 -- Different random result each time, should be 100,000
(1 row)
Thoughts?
Regards,
Dean
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