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From: Dean Rasheed <dean.a.rasheed@gmail.com>
Date: Tue, 4 Sep 2018 08:21:07 +0100
Message-ID: 
 <CAEZATCVy77hhgTZ1Yrvbp6GbsUuWY0Bw8CLbv_Jr2mAF_bOkBA@mail.gmail.com>
Subject: Re: [HACKERS] proposal: schema variables
To: Pavel Stehule <pavel.stehule@gmail.com>
Cc: Fabien COELHO <coelho@cri.ensmp.fr>,
 Gilles Darold <gilles.darold@dalibo.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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