I wrote: >> I have a slight reservaton about whether 1.25x is still a sensible >> heuristic. > > This was also discussed in [1], but no patch came out of it. I was > just now turning the formulas discussed there into code, but I'll > defer to someone with more expertise. FWIW, I suspect that a solution > that doesn't take into account a metric like coefficient of variation > will have the wrong behavior sometimes, whether for highly uniform or > highly non-uniform distributions.
I spent a few hours hacking on this, and it turns out calculating the right number of MCVs taking into account both uniform and highly non-uniform distributions is too delicate a problem for me to solve right now. The logic suggested by Dean Rasheed in [1] always produces no MCVs for a perfectly uniform distribution (which is good), but very often also for other distributions, which is not good. My efforts to tweak that didn't work, so I didn't get as far as adapting it for the problem Jeff is trying to solve. I have not been able to come up with a more compelling alternative, so I have nothing further to say about the patch under review. > [1] > https://www.postgresql.org/message-id/flat/32261.1496611829%40sss.pgh.pa.us#32261.1496611...@sss.pgh.pa.us -John Naylor
