Re: Logic behind parallel default? WAS: Rename max_parallel_degree?

Tom Lane <tgl@sss.pgh.pa.us>

From: Tom Lane <tgl@sss.pgh.pa.us>

To: Josh berkus <josh@agliodbs.com>

Cc: Peter Geoghegan <pg@heroku.com>, Robert Haas <robertmhaas@gmail.com>, Noah Misch <noah@leadboat.com>, David Rowley <david.rowley@2ndquadrant.com>, Amit Kapila <amit.kapila16@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Magnus Hagander <magnus@hagander.net>, Bruce Momjian <bruce@momjian.us>, PostgreSQL-development <pgsql-hackers@postgresql.org>

Date: 2016-05-31T18:57:34Z

Lists: pgsql-hackers

Josh berkus <josh@agliodbs.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/20160420174631.3qjjhpwsvvx5bau5@alap3.anarazel.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