Re: [HACKERS] proposal: schema variables

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: Dean Rasheed <dean(dot)a(dot)rasheed(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: 2018-09-14 21:31:16
Message-ID: CAFj8pRDREykg0hnMBkryBmSTtqEs9QUKUp_447_F5P1j8bkH1w@mail.gmail.com
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út 4. 9. 2018 v 9:21 odesílatel Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
napsal:

> 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?
>

This issue should be fixed in attached patch (and more others).

The code is more cleaner now, there are more tests, and documentation is
mostly complete. I am sorry - my English is not good.
New features:

o ON COMMIT DROP and ON TRANSACTION END RESET -- remove temp variable on
commit, reset variable on transaction end (commit, rollback)
o LET var = DEFAULT -- reset specified variable

Regards

Pavel

> Regards,
> Dean
>

Attachment Content-Type Size
schema-variables-20180914-01.patch text/x-patch 235.6 KB

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