From: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
---|---|
To: | James Sewell <james(dot)sewell(at)lisasoft(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Paul Ramsey <pramsey(at)cleverelephant(dot)ca>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Parallel Aggregate |
Date: | 2016-03-15 01:18:51 |
Message-ID: | CAKJS1f_itLNeVB9uV4GYiBrRZxtpVVyqRSGjKgW+W3AsA1oH8w@mail.gmail.com |
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Lists: | pgsql-hackers |
On 15 March 2016 at 11:24, James Sewell <james(dot)sewell(at)lisasoft(dot)com> wrote:
> On Tuesday, 15 March 2016, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>
>> > Does the cost of the aggregate function come into this calculation at
>> > all? In PostGIS land, much smaller numbers of rows can generate loads
>> > that would be effective to parallelize (worker time much >> than
>> > startup cost).
>>
>> Unfortunately, no - only the table size. This is a problem, and needs
>> to be fixed. However, it's probably not going to get fixed for 9.6.
>> :-(
>
>
> Any chance of getting a GUC (say min_parallel_degree) added to allow setting
> the initial value of parallel_degree, then changing the small relation check
> to also pass if parallel_degree > 1?
>
> That way you could set min_parallel_degree on a query by query basis if you
> are running aggregates which you know will take a lot of CPU.
>
> I suppose it wouldn't make much sense at all to set globally though, so it
> could just confuse matters.
I agree that it would be nice to have more influence on this decision,
but let's start a new thread for that. I don't want this one getting
bloated with debates on that. It's not code I'm planning on going
anywhere near for this patch.
I'll start a thread...
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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