Re: More stable query plans via more predictable column statistics

Alex Shulgin <alex(dot)shulgin(at)gmail(dot)com> writes:
> On Sun, Apr 3, 2016 at 7:18 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Well, we have to do *something* with the last (possibly only) value.
>> Neither "include always" nor "omit always" seem sane to me. What other
>> decision rule do you want there?

> Well, what implies that the last value is somehow special? I would think
> we should just do with it whatever we do with the rest of the candidate
> MCVs.

Sure, but both of the proposed decision rules break down when there are no
values after the one under consideration. We need to do something sane
there.

> For "the only value" case: we cannot build a histogram out of a single
> value, so omitting it from MCVs is not a good strategy, ISTM.
> From my point of view that amounts to "include always".

If there is only one value, it will have 100% of the samples, so it would
get included under just about any decision rule (other than "more than
100% of this value plus following values"). I don't think making sure
this case works is sufficient to get us to a reasonable rule --- it's
a necessary case, but not a sufficient case.

regards, tom lane