Re: How to hint 2 coulms IS NOT DISTINCT FROM each other

On Fri, Oct 28, 2016 at 1:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> writes:
>> On Fri, Oct 28, 2016 at 10:29 AM, Kim Rose Carlsen <krc(at)hiper(dot)dk> wrote:
>>> I was wondering if there is a way to hint that two columns in two different
>>> tables IS NOT DISTINCT FROM each other. So that the optimizer may assume if
>>> table_a.key = 'test' THEN table_b.key = 'test' .
>>>
>>> The equals operator already does this but it does not handle NULLS very well
>>> (or not at all). And it seems IS NOT DISTINCT FROM is not indexable and
>>> doesn't establish the same inference rules as equals.
>
>> The whole idea behing Postgres' query planner is that you don't have
>> to use any hints. Late model versions of postgres handle nulls fine,
>> but nulls are never "equal" to anything else. I.e. where xxx is null
>> works with indexes. Where x=y does not, since null <> null.
>
> The bigger picture here is that if you've designed a data representation
> that requires that a null be considered "equal to" another null, you're
> really going to be fighting against the basic semantics of SQL. You'd
> be best off to rethink the representation. We've not seen enough info
> about your requirements to suggest just how, though.

Well, ISTM OP is simply asking, "why can't IS [NOT] DISTINCT FROM be
optimized"? It ought to be, at least in some cases. Internally
indexes handle nulls so you should be able to implement them to
satisfy those kinds of scans. I guess that's an easy thing to say
though.

merlin