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
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