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. Because of the condition "b.col2 =
1", which implies that "b.col1 IS NOT NULL", that's actually equal to:
select a.col2
from a inner join b on a.col1 = b.col1
where b.col2 = 1;
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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