Horton

Re: [HACKERS] Vacuum: allow usage of more than 1GB of work mem

Claudio Freire <klaussfreire@gmail.com>

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From: Claudio Freire <klaussfreire@gmail.com>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>, Thomas Munro <thomas.munro@enterprisedb.com>, Stephen Frost <sfrost@snowman.net>, Michael Paquier <michael.paquier@gmail.com>, Daniel Gustafsson <daniel@yesql.se>, Andres Freund <andres@anarazel.de>, Masahiko Sawada <sawada.mshk@gmail.com>, Anastasia Lubennikova <a.lubennikova@postgrespro.ru>, Anastasia Lubennikova <lubennikovaav@gmail.com>, PostgreSQL-Dev <pgsql-hackers@postgresql.org>
Date: 2018-02-08T03:13:09Z
Lists: pgsql-hackers 
On Wed, Feb 7, 2018 at 11:29 PM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> Claudio Freire wrote:
>> On Wed, Feb 7, 2018 at 8:52 PM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>> >> Waiting as you say would be akin to what the patch does by putting
>> >> vacuum on its own parallel group.
>> >
>> > I don't think it's the same.  We don't need to wait until all the
>> > concurrent tests are done -- we only need to wait until the transactions
>> > that were current when the delete finished are done, which is very
>> > different since each test runs tons of small transactions rather than
>> > one single big transaction.
>>
>> Um... maybe "lock pg_class" ?
>
> I was thinking in first doing
>   SELECT array_agg(DISTINCT virtualtransaction) vxids
>     FROM pg_locks \gset
>
> and then in a DO block loop until
>
>    SELECT DISTINCT virtualtransaction
>      FROM pg_locks
> INTERSECT
>    SELECT (unnest(:'vxids'::text[]));
>
> returns empty; something along those lines.

Isn't it the same though?

I can't think how a transaction wouldn't be holding at least an access
share on pg_class.

Commits

  1. Prefetch blocks during lazy vacuum's truncation scan

  2. Explain unaccounted for space in pgstattuple.