Re: Unable to use index?

Edmund Dengler <edmundd@eSentire.com> writes:
> Hmm, interesting as I have that table clustered starting with the
> rep_component, so 'ps_probe' will definitely appear later in a sequential
> scan. So why does the <order by> force the use of the index?

It does not "force" anything, it simply alters the cost estimates.  The
seqscan-based plan requires an extra sort step to meet the ORDER BY,
while the indexscan plan does not.  In this particular scenario the
indexscan plan is estimated to beat seqscan+sort, but in other cases the
opposite decision might be made.

            regards, tom lane