Simon Riggs wrote:
> On Tue, 2008-09-02 at 13:20 +0300, Heikki Linnakangas wrote:
>> Simon Riggs wrote:
>>> It turns out that a join like this
>>>
>>> select a.col2
>>> from a left outer join b on a.col1 = b.col1
>>> where b.col2 = 1;
>>>
>>> can be cheaper if we don't remove the join, when there is an index on
>>> a.col1 and b.col2, because the presence of b allows the values returned
>>> from b to be used for an index scan on a.
>> Umm, you *can't* remove that join.
>
> Yes, you can. The presence or absence of rows in b is not important to
> the result of the query because of the "left outer join".
>
> I spent nearly a whole day going down that deadend also.
Oh. How does the query look like after removing the join, then?
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com