On Mon, 6 Jul 2020 at 12:37, Robins Tharakan <thara...@gmail.com> wrote:
> When an SQL needs to UNION constants on either side, it should be possible > to > implicitly apply a LIMIT 1 and get good speed up. Is this an incorrect > understanding, > or something already discussed but rejected for some reason? > > This need came up while reviewing generated SQL, where the need was to > return true when > at least one of two lists had a row. A simplified version is given below: > > (SELECT 1 FROM pg_class) UNION (SELECT 1 FROM pg_class); > vs. > (select 1 FROM pg_class limit 1) UNION (SELECT 1 FROM pg_class limit 1); > -- Faster > Those two queries aren't logically equivalent, so you can't apply the LIMIT 1 as an optimization. First query returns lots of random rows, the second query returns just one random row. -- Simon Riggs http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/> Mission Critical Databases
