Re: WIP Join Removal

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