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help / color / mirror / Atom feedFrom: Isaac Morland <[email protected]>
To: Tom Lane <[email protected]>
Cc: David G. Johnston <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Yura Sokolov <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: When IMMUTABLE is not.
Date: Thu, 15 Jun 2023 10:55:01 -0400
Message-ID: <CAMsGm5fZWHTn4sugZ02ZaxBjPC88=Z9a3zF-mgTk6FHX_0zqxA@mail.gmail.com> (raw)
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On Thu, 15 Jun 2023 at 10:49, Tom Lane <[email protected]> wrote:
In particular, we've never enforced that an immutable function can't
> call non-immutable functions. While that would seem like a good idea
> in the abstract, we've intentionally not tried to do it. (I'm pretty
> sure there is more than one round of previous discussions of the point
> in the archives, although locating relevant threads seems hard.)
> One reason not to is that polymorphic functions have to be marked
> with worst-case volatility labels. There are plenty of examples of
> functions that are stable for some input types and immutable for
> others (array_to_string, for instance); but the marking system can't
> represent that so we have to label them stable. Enforcing that a
> user-defined immutable function can't use such a function might
> just break things for no gain.
>
More sophisticated type systems (which I am *not* volunteering to graft
onto Postgres) can handle some of this, but even Haskell has
unsafePerformIO. The current policy is both wise and practical.
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Subject: Re: When IMMUTABLE is not.
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