Geoff Winkless <pgsqladmin(at)geoff(dot)dj> writes:
> But as far as I can see, apart from the absolute extremes, the
> index-only scan is _always_ going to be quicker than the index+table
> scan.
Well, that is a different issue: what does the planner think of an
index-only scan as compared to a regular index scan. I suspect that
it's pricing the IOS very high because a lot of the table is dirty
and therefore will have to be visited even in a nominally index-only
scan. You might check whether the plan choice changes immediately
after a VACUUM of the table.
regards, tom lane