Robert Haas <robertmh...@gmail.com> writes: > On Fri, Mar 17, 2017 at 1:14 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> It would not be too hard to convince me that neqjoinsel should >> simply return 1.0 for any semijoin/antijoin case, perhaps with >> some kind of discount for nullfrac. Whether or not there's an >> equal row, there's almost always going to be non-equal row(s). >> Maybe we can think of a better implementation but that seems >> like the zero-order approximation.
> Yeah, it's not obvious how to do better than that considering only one > clause at a time. Of course, what we really want to know is > P(x<>y|z=t), but don't ask me how to compute that. Yeah. Another hole in this solution is that it means that the estimate for x <> y will be quite different from the estimate for NOT(x = y). You wouldn't notice it in the field unless somebody forgot to put a negator link on their equality operator, but it seems like ideally we'd think of a solution that made sense for generic NOT in this context. No, I have no idea how to do that. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers