On Sun, Apr 3, 2016 at 7:49 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Alex Shulgin <alex.shul...@gmail.com> writes: > > On Sun, Apr 3, 2016 at 7:18 AM, Tom Lane <t...@sss.pgh.pa.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. >
Hm... There are indeed the case where it would beneficial to have at least 2 values in the histogram (to have at least the low/high bounds for inequality comparison selectivity) instead of taking both to the MCV list or taking one to the MCVs and having to discard the other. Obviously, we need a fresh idea on how to handle this. > 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. > Well, if it's the only value it will be accepted simply because we are checking that special case already and don't even bother to loop through the track list. -- Alex
