Re: More stable query plans via more predictable column statistics

On Sun, Apr 3, 2016 at 7:18 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Alex Shulgin <alex(dot)shulgin(at)gmail(dot)com> writes:
> > On Sun, Apr 3, 2016 at 3:43 AM, Alex Shulgin <alex(dot)shulgin(at)gmail(dot)com>
> wrote:
> >> I'm not sure yet about the 1% rule for the last value, but would also
> love
> >> to see if we can avoid the arbitrary limit here. What happens with a
> last
> >> value which is less than 1% popular in the current code anyway?
>
> > Now that I think about it, I don't really believe this arbitrary
> heuristic
> > is any good either, sorry.
>
> Yeah, it was just a placeholder to produce a working patch.
>
> Maybe we could base this cutoff on the stats target for the column?
> That is, "1%" would be the right number if stats target is 100,
> otherwise scale appropriately.
>
> > What was your motivation to introduce some limit at the bottom anyway?
>
> 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.

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". What problems do
you see with this approach exactly?

--
Alex