From: Tom Lane <tgl@sss.pgh.pa.us>
To: Ken Tanzer <ken.tanzer@gmail.com>
cc: pgsql-performance@lists.postgresql.org
Subject: Re: Speeding up query pulling comments from pg_catalog
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Comments: In-reply-to Ken Tanzer <ken.tanzer@gmail.com>
	message dated "Sat, 20 Jul 2019 12:08:00 -0700"
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Date: Sat, 20 Jul 2019 15:25:03 -0400
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Ken Tanzer <ken.tanzer@gmail.com> writes:
> On Sat, Jul 20, 2019 at 7:46 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> and then to add insult to injury, has to search pg_description a second
>> time for each hit.

> Not sure if I'm understanding this correctly, but are you saying that
> because col_description() is specified in two places in the query, that it
> actually will get called twice?

Yes.

> I was under the impression that a function
> (at least a non-volatile one) specified multiple times, but with the same
> arguments, would only get called once. Is that just wishful thinking?

Afraid so.  There's been assorted talk about various optimizations to
avoid unnecessary duplicate function calls, but I don't think that
merging textually-separate calls has even been on the radar.  The
discussions I can recall have been more about not calling stable functions
(with fixed arguments) more than once per query.

			regards, tom lane