On Thu, 12 May 2005 17:40:06 -0400, Tom Lane <tgl@sss.pgh.pa.us>
wrote:
>the planner believes that only
>consecutive columns in the index are usable --- that is, if you have
>quals for a and c but not for b, it will think that the condition for c
>isn't usable with the index. This is true for btree [...]
It's not difficult to setup a test case where an index is used, but
only with a=42 as an index condition, and c='foo' is applied as a
filter condition. Now add a redundant condition on b... AND b BETWEEN minb AND maxb ...
and watch how c='foo' moves into the index condition.
I did this test some time ago and I believe that adding the condition
on b did not change the estimated cost, only the actual execution
time.
ServusManfred