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)
But anyway, Greg's example looks valid, and proves the point that
removing a join isn't always a win.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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