Robert Haas <robertmh...@gmail.com> writes: > On Fri, Jul 1, 2016 at 9:52 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> Maybe, but neither UNION nor UNION ALL would duplicate the semantics >> of OR, so there's some handwaving here that I missed.
> SELECT * FROM foo WHERE a = 5 OR a = 4 > isn't equivalent to > SELECT * FROM foo WHERE a = 5 > UNION > SELECT * FROM foo WHERE a = 4 > ? It probably is, but you're assuming that "a" appears in the list of columns being unioned. If you make that just "SELECT b FROM ..." then the latter form gets rid of duplicate b values where the first doesn't. On the other hand, UNION ALL might introduce duplicates not present in the OR query's result. 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