Re: [18] Policy on IMMUTABLE functions and Unicode updates

Joe Conway <mail(at)joeconway(dot)com> writes:
> So you are proposing we add STATIC to VOLATILE/STABLE/IMMUTABLE (in the
> third position before IMMUTABLE), give it IMMUTABLE semantics, mark
> builtin functions that deserve it, and document with suitable caution
> statements?

What is the point of that, exactly?

I'll agree that the user documentation could use some improvement
in how it describes the volatility levels, but I do not see how
it will reduce anybody's confusion to invent multiple aliases for
what's effectively the same volatility level. Nor do I see a
use-case for actually having multiple versions of "immutable".
Once you've decided you can put something into an index, quibbling
over just how immutable it is doesn't really change anything.

To put this another way: the existing volatility levels were
basically reverse-engineered from the ways that the planner could
meaningfully treat a function: it's dangerous, it is safe enough
to use in an index condition (which changes the number of times
the query will evaluate it), or it's safe to constant-fold in
advance of execution. Unless there's a fourth planner behavior that's
worth having, we don't need a fourth level. Possibly you could
argue that "safe to put in an index" is a different level from
"safe to constant-fold", but I don't really agree with that.

regards, tom lane