On Thu, 15 Jun 2023 at 10:49, Tom Lane <t...@sss.pgh.pa.us> 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.
