Hadi Moshayedi <hadi@citusdata.com> writes:
> I am wondering why is it not using index-only-scan (which would use the
> cache better) and instead it does a bitmap scan?
Never experiment on an empty table and assume that the resulting plan
is the same as you'd get on a large table.
In this case, not only don't you have any meaningful amount of data
loaded, but the planner can see that none of the table's pages are
marked all-visible, meaning that the "index-only" scan would degrade
to a regular indexscan, which is how it gets costed. And on a single-page
table, an indexscan is going to have a hard time beating other
alternatives.
regards, tom lane