From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: select_parallel test fails with nonstandard block size |
Date: | 2016-09-15 14:11:16 |
Message-ID: | CA+TgmoZ2w1=6KkpSpdMSxkrDezWXSq5t37HhnZ_Syj4d0MfRvA@mail.gmail.com |
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On Thu, Sep 15, 2016 at 9:59 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Possibly we ought to change things so that the default value of
> min_parallel_relation_size is a fixed number of bytes rather
> than a fixed number of blocks. Not sure though.
The reason why this was originally reckoned in blocks is because the
data is divided between the workers on the basis of a block number.
In the degenerate case where blocks < workers, the extra workers will
get no blocks at all, and thus no rows at all. It seemed best to
insist that the relation had a reasonable number of blocks so that we
could hope for a reasonably even distribution of work among a pool of
workers. I'm not altogether sure that's the right way of thinking
about this problem but I'm not sure it's wrong, either; anyway, it's
as far as my thought process had progressed at the time I wrote the
code.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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