From: Tom Lane <tgl@sss.pgh.pa.us>
To: chap@anastigmatix.net
cc: Yura Sokolov <y.sokolov@postgrespro.ru>,
        pgsql-hackers@lists.postgresql.org
Subject: Re: When IMMUTABLE is not.
In-reply-to: <69234e25e34e68425f49630f79ef9991@anastigmatix.net>
References: <389c986d-fbb4-c644-9280-db7836af7ca9@postgrespro.ru> <2111925.1686835298@sss.pgh.pa.us> <69234e25e34e68425f49630f79ef9991@anastigmatix.net>
Comments: In-reply-to chap@anastigmatix.net
	message dated "Thu, 15 Jun 2023 09:58:39 -0400"
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Date: Thu, 15 Jun 2023 10:10:03 -0400
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chap@anastigmatix.net writes:
> And also, isn't it the case that IMMUTABLE should mark a function,
> not merely that "doesn't manipulate data", but whose return value
> doesn't depend in any way on data (outside its own arguments)?

Right.  We can't realistically enforce that either, so it's
up to the user.

> The practice among PLs of choosing an SPI readonly flag based on
> the IMMUTABLE/STABLE/VOLATILE declaration seems to be a sort of
> peculiar heuristic, not something inherent in what that declaration
> means to the optimizer. (And also influences what snapshot the
> function is looking at, and therefore what it can see, which has
> also struck me more as a tacked-on effect than something inherent
> in the declaration's meaning.)

Well, it is a bit odd at first sight, but these properties play
together well.  See

https://www.postgresql.org/docs/current/xfunc-volatility.html

			regards, tom lane