Re: Optimize constant MinMax expressions

Vik Fearing <vik(dot)fearing(at)2ndquadrant(dot)com> writes:
> I was working on a little thing where I needed to simulate BETWEEN
> SYMMETRIC so naturally I used least() and greatest(). I was a little
> surprised to see that my expressions were not folded into straight
> constants and the estimates were way off as a consequence.

> I came up with the attached patch to fix it, but it's so ridiculously
> small that I fear I'm missing something.

Well, the question this is begging is in the adjacent comment:

* Generic handling for node types whose own processing is
* known to be immutable, and for which we need no smarts

Can we assume that the underlying datatype comparison function is
immutable? I guess so, since we assume that in nearby code such as
contain_mutable_functions_walker, but I don't think it should be done
without at least a comment.

BTW, poking around for other code involving MinMaxExpr, I notice that
contain_leaked_vars_walker is effectively assuming that all datatype
comparison functions are leakproof, an assumption I find a bit debatable.
Maybe it's all right, but again, it should certainly not have gone without
a comment.

regards, tom lane