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help / color / mirror / Atom feedFrom: Pavel Stehule <pavel.stehule@gmail.com>
To: Dean Rasheed <dean.a.rasheed@gmail.com>
Cc: Fabien COELHO <coelho@cri.ensmp.fr>
Cc: Gilles Darold <gilles.darold@dalibo.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: [HACKERS] proposal: schema variables
Date: Tue, 4 Sep 2018 15:00:51 +0200
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Hi
2018-09-04 9:21 GMT+02:00 Dean Rasheed <dean.a.rasheed@gmail.com>:
> 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?
>
The query use copy of values of variables now - but unfortunately, these
values are not passed to workers. Should be fixed.
Thank you for test case.
Pavel
> Regards,
> Dean
>
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Cc: pavel.stehule@gmail.com, dean.a.rasheed@gmail.com, coelho@cri.ensmp.fr, gilles.darold@dalibo.com, pgsql-hackers@lists.postgresql.org
Subject: Re: [HACKERS] proposal: schema variables
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