| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Josh berkus <josh(at)agliodbs(dot)com> |
| Cc: | Peter Geoghegan <pg(at)heroku(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Logic behind parallel default? WAS: Rename max_parallel_degree? |
| Date: | 2016-05-31 18:57:34 |
| Message-ID: | 23351.1464721054@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Josh berkus <josh(at)agliodbs(dot)com> writes:
> On 05/31/2016 11:10 AM, Tom Lane wrote:
>> The 9.6 open-items list cites
>> https://www.postgresql.org/message-id/flat/20160420174631(dot)3qjjhpwsvvx5bau5(at)alap3(dot)anarazel(dot)de
> Looks like we didn't decide for the release, just the beta.
Indeed. I think it's premature to have this discussion. The plan
was to evaluate near the end of beta, when we (hopefully) have a
better feeling for how buggy parallel query is likely to be.
> Also, defaulting to off lets users make more use of the parallel_degree
> table attribute to just enable parallelism on select tables.
Well, that's an interesting point. The current coding is that
parallel_degree is an upper limit on per-table workers, and
max_parallel_degree also limits it. So if you want parallel scans only on
a small set of tables, parallel_degree is not an especially convenient way
to get to that. Whether we measure it in workers or cores doesn't change
this conclusion.
It might be worth reconsidering what per-table knobs we should provide
exactly, but that's orthogonal to the main point under discussion.
regards, tom lane
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