am Thu, dem 16.10.2008, um 14:43:42 +0300 mailte Andrus folgendes: > I'm looking a hint for new application dynamic query builder creation for > PostgreSQL 8.0+ > > Following two queries return same results: > > SELECT ... > FROM t1 JOIN t2 USING (cx) > LEFT JOIN t3 USING (cy) > LEFT JOIN t4 USING (cz) > WHERE ... > > and > > SELECT ... > FROM (SELECT * FROM t1 JOIN t2 USING (cx) LEFT JOIN t3 USING (cy) WHERE > ... ) p1 > LEFT JOIN t4 USING (cz) > WHERE ... > > Second query is easier to generate but contains two where clauses which > produce logically same resultset and in first query. > So it is preferable to use second form. However I'm not clear how this > affects query perfomance. > > Questions: > > Will execution speed of both queries be the same ? > Will 8.0+ optimizers create same execution plan for those queries ? You can check this with EXPLAIN ANALYSE <your_query>. I'm guessing it depends on the PG-Version, recent versions are smarter. Andreas -- Andreas Kretschmer Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header) GnuPG-ID: 0x3FFF606C, privat 0x7F4584DA http://wwwkeys.de.pgp.net -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
