Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> It still seems like awfully weird behavior. > > Why? The WHERE condition relates only to the output of the _stats > subquery, so why shouldn't it be evaluated there, rather than > after the join?
In another thread, Tom Lane <t...@sss.pgh.pa.us> wrote: > It's easier to understand why this is if you realize that SQL has > a very clear model of a "pipeline" of query execution. > Conceptually, what happens is: > > 1. Form the cartesian product of the tables listed in FROM (ie, > all combinations of rows). > > 2. Apply the WHERE condition to each row from 1, and drop rows > that don't pass it. People expect that the results will be consistent with this model, even if the implementation is optimized "under the covers". I think correct semantics should trump performance here. -Kevin -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
