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From: Thomas Munro <[email protected]>
To: Marc SCHAEFER <[email protected]>
Cc: [email protected]
Subject: Re: Naive question about multithreading/multicore
Date: Sun, 13 Oct 2024 08:16:04 +1300
Message-ID: <CA+hUKG+bjgdh7APFCC_8cZKnMzeagYjiEqTp+qn2koafGmi1aQ@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

On Sun, Oct 13, 2024 at 6:31 AM Marc SCHAEFER
<[email protected]> wrote:
> template1=> SELECT COUNT(*) FROM pg_class a, pg_class b, pg_class c;
>
> I see only one 100% CPU PostgreSQL process.

If you set set min_parallel_table_scan_size = 0 then it uses
parallelism, and completes much faster.  The planner generally works
by comparing the estimated cost of various plans (it is a "cost based"
optimiser), but the decision to actually consider parallelism at all
is essentially "rule based", and the rules aren't smart enough for
this query with default settings.  pg_class is considered too small to
bother parallelising the scan, and here you have a 3-way cross-join
which generates an enormous of work for each tuple so it is actually
a good idea to parallelise it.  I guess people don't actually do that too
often.

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