It says here that CREATE CAST insists the cast function be immutable.
This seems wrong to me, in view of the fact that we have numerous
built-in casts that don't adhere to that rule --- for example,
timestamptz(date) is not immutable because it depends on the timezone
setting.
Perhaps there's a case for prohibiting volatile casts (as opposed to
stable ones), but I don't really see it. I'd prefer to just remove
this restriction. Comments?
regards, tom lane