Re: When IMMUTABLE is not.

chap(at)anastigmatix(dot)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