On Tue, 2008-09-02 at 14:20 +0300, Heikki Linnakangas wrote: > Simon Riggs wrote: > > select a.col2 > > from a left outer join b on a.col1 = b.col1 > > where b.col2 = 1; > > > > is logically equivalent to > > > > select a.col2 > > from a; > > No, it's not: > > postgres=# CREATE TABLE a (col1 int4, col2 int4); > CREATE TABLE > postgres=# CREATE TABLE b (col1 int4, col2 int4); > CREATE TABLE > postgres=# INSERT INTO a VALUES (1,1); > INSERT 0 1 > postgres=# select a.col2 from a; > col2 > ------ > 1 > (1 row) > > postgres=# select a.col2 from a left outer join b on a.col1 = b.col1 > where b.col2 = 1; > col2 > ------ > (0 rows)
You raise an interesting and important point that shows an error of mine. Notice that select a.col2 from a left outer join b on a.col1 = b.col1 *and* b.col2 = 1; can be re-written as select a.col2 from a; whereas select a.col2 from a left outer join b on a.col1 = b.col1 where b.col2 = 1; cannot, as you show. It seems I wrote my original tests using "and" instead of "where" and hadn't noticed the distinction. Thanks for helping me catch that error. I will put back the code that looks for an empty filter condition on the checkrel. That day was not wasted after all. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-patches mailing list (pgsql-patches@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-patches
