From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
Cc: | Daniel Verite <daniel(at)manitou-mail(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Magnus Hagander <magnus(at)hagander(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Rename max_parallel_degree? |
Date: | 2016-05-02 16:52:55 |
Message-ID: | CA+TgmoZqTN0UnY7AH0NR=dG7G847NmGyUcids_N8yjQMKcSSyQ@mail.gmail.com |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
On Sat, Apr 30, 2016 at 4:54 AM, David Rowley
<david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>> Right, but they're probably not doing the SAME work. You can look at
>> EXPLAIN (ANALYZE, VERBOSE, BUFFERS) to see. Of course, all the work
>> above the Gather node is being done by the leader, but the stuff below
>> the Gather node often has a bit of participation from the leader, but
>> is mostly the workers.
>
> Robert, I'd imagine that most of your tests to make you think what you
> do would have come from testing parallel seq scan, where perhaps
> Daniel's comes from testing something like parallel aggregates, or at
> least something that gives the workers a decent amount of work per
> tuple returned.
Sure, if you're doing parallel aggregate with a small number of output
groups, the work distribution will be much more nearly equal. I
wasn't trying to dispute that. There are many cases where it doesn't
work out that way, though.
> So I'd say this very much depends on how busy the main process is
> pulling rows from each worker.
Absolutely.
> It would also be quite nice if we could see at a glance how much the
> main process did, without having to go subtracting what all the
> workers managed to do.
I agree, but that looked considerably more invasive, because of the
way that the worker results get aggregated into the leader's result.
There may be a clever way to avoid that, but I didn't see it. Or we
may want to do it anyway despite it requiring more invasive changes.
I just wasn't in a hurry to rush into all that while trying to get
parallel query v1 out the door.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
From | Date | Subject | |
---|---|---|---|
Next Message | Andres Freund | 2016-05-02 16:54:38 | Re: [BUGS] Breakage with VACUUM ANALYSE + partitions |
Previous Message | Robert Haas | 2016-05-02 16:44:53 | Re: [BUGS] Breakage with VACUUM ANALYSE + partitions |