Josh Berkus <josh@xxxxxxxxxxxx> writes: >> Personally, I'll bet lunch that that external software is outright >> broken, ie it probably thinks "X = X" is constant true and they found >> they could save two lines of code and a few machine cycles by emitting >> that rather than not emitting anything. > Well, it was more in the form of: > tab1.x = COALESCE(tab2.y,tab1.x) Hm. I'm not following how you get from there to complaining about not being smart about X = X, because that surely ain't the same. > Well, I'd be more satisfied with having a solution for: > WHERE tab1.x = tab1.y > ... in general, even if it didn't have correlation stats. Like, what's > preventing us from using the same selectivity logic we would on a join > for that? It's a totally different case. In the join case you expect that each element of one table will be compared with each element of the other. In the single-table case, that's exactly *not* what will happen, and I don't see how you get to anything very useful without knowing something about the value pairs that actually occur. As a concrete example, applying the join selectivity logic would certainly give a completely wrong answer for X = X, unless there were only one value occurring in the column. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
