On Fri, Jul 1, 2016 at 10:11 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > 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.
Right, so, significant query transformations are non-trivial. But the point is that with the upper planification stuff, I think it is possible, at least in some cases, that we could consider reordering set operations with scan/join planning, just as we've previously talked about reordering grouping stages relative to scan/join planning. The details are undeniably hard to get right. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers