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From: Tom Lane <[email protected]>
To: [email protected]
Cc: Yura Sokolov <[email protected]>
Cc: [email protected]
Subject: Re: When IMMUTABLE is not.
Date: Thu, 15 Jun 2023 10:10:03 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

[email protected] 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

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