Subject: Re: Rename max_parallel_degree?
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
 Peter Geoghegan <pg@heroku.com>
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Cc: Tom Lane <tgl@sss.pgh.pa.us>, 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>
From: Josh berkus <josh@agliodbs.com>
Message-ID: <574DD64A.9090600@agliodbs.com>
Date: Tue, 31 May 2016 11:22:02 -0700
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On 05/31/2016 11:17 AM, Peter Eisentraut wrote:
> On 5/31/16 2:02 PM, Josh berkus wrote:
>> I get where you're coming from, but I think Haas's query plan output is
>> going to show us the confusion we're going to get.  So we need to either
>> change the parameter, the explain output, or brace ourselves for endless
>> repeated questions.
> 
> Changing the explain output doesn't sound so bad to me.
> 
> The users' problem is that the parameter setting ought to match the
> EXPLAIN output.
> 
> The developers' problem is that the EXPLAIN output actually corresponds
> to leader + (N-1) workers internally.
> 
> I think we can hope that developers are going to be less confused about
> that than users.

Makes sense.

One more consistency question: what's the effect of running out of
max_parallel_workers?

That is, say max_parallel_workers is set to 10, and 8 are already
allocated.  If I ask for max_parallel_X = 4, how many cores to I use?

Presumably the leader isn't counted towards max_parallel_workers?

-- 
--
Josh Berkus
Red Hat OSAS
(any opinions are my own)


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