Re: ALTER TYPE 3: add facility to identify further no-work cases

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> Oh, really? I was thinking the logic should go into find_coercion_pathway().

Well, I've been saying right along that it should be in
eval_const_expressions. Putting this sort of optimization in the parser
is invariably the wrong thing, because it fails to catch all the
possibilities. As an example, inlining a SQL function could expose
opportunities of this type. Another issue is that premature
optimization in the parser creates headaches if conditions change such
that a previous optimization is no longer valid --- you may have stored
rules wherein the optimization was already applied. (Not sure that
specific issue applies to casting, since we have no ALTER CAST commmand;
but in general you want expression optimizations applied downstream from
the rule rewriter not upstream.)

regards, tom lane