Re: Optimize join selectivity estimation by not reading MCV stats for unique join attributes

I wrote:
> Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
>> Or perhaps what if we have a function that quickly determines if the
>> attribute has MCV, without loading it? I'd bet the expensive part of
>> get_attstatslot() is the deconstruct_array().
>> We could have a function that only does the first small loop over slots,
>> and returns true/false if we have a slot of the requested stakind.

> Yeah, I like this idea.

Actually, looking at get_attstatslot, I realize it was already designed
to do that -- just pass zero for flags. So we could do it as attached.

We could make some consequent simplifications by only retaining one
"have_mcvs" flag, but I'm inclined to leave the rest of the code as-is.
We would not get much gain from that, and it would make this harder
to undo if there ever is a reason to consider just one set of MCVs.

regards, tom lane