Re: Proposition to use '==' as synonym for 'IS NOT DISTINCT FROM'

Andres Freund <andres@anarazel.de> writes:
> On 2019-10-26 14:23:49 -0400, Tom Lane wrote:
>> ... instead of
>>     x IS NOT DISTINCT FROM y
>> I'm vaguely imagining
>>     x = {magic} y
>> where unlike Eugen's suggestion, "=" is the real name of the underlying
>> comparison operator.  For dump/restore this could be spelled verbosely
>> as
>>     x OPERATOR(someplace.=) {magic} y
>> The hard part is to figure out some {magic} annotation that is both
>> short and unambiguous.  We have to cover the IS DISTINCT variant, too.

To clarify, what I have in mind here doesn't have any effect whatever
on the parse tree or the execution semantics, it's just about offering
an alternative SQL textual representation.

> Leaving the exact choice of how {magic} would look like, are you
> thinking of somehow making it work for every operator, or just for some
> subset? It's intriguing to have something generic, but I'm not quite
> clear how that'd would work?  It's not clear to me how we'd
> automatically infer a sensible meaning for e.g. < etc.

Yeah, I think it could only be made to work sanely for underlying
operators that have the semantics of equality.  The NOT DISTINCT
wrapper has the semantics

    NULL vs NULL        -> true
    NULL vs not-NULL    -> false
    not-NULL vs NULL    -> false
    not-NULL vs not-NULL    -> apply operator

and while theoretically the operator needn't be equality, those
NULL behaviors don't make much sense otherwise.  (IS DISTINCT
just inverts all the results, of course.)

I suppose that we could also imagine generalizing DistinctExpr
into something that could work with other operator semantics,
but as you say, it's a bit hard to wrap ones head around what
that would look like.

> And even if we just restrict it to = (and presumably <> and !=), in
> which cases is this magic going to work? Would we tie it to the textual
> '=', '<>' operators? btree opclass members?

See the other thread I cited --- right now, the underlying operator is
always "=" and it's looked up by name.  Whether that ought to change
seems like a separate can o' worms.

            regards, tom lane