On Tue, 20 Feb 2001, Joseph Shraibman wrote:
> Stephan Szabo wrote:
>
> > Where are you seeing something that says the estimator/planner using the
> > index to get an upper bound? The estimator shouldn't be asking either the
> > index or the heap for anything, it should be working entirely with the
> > statistics that were generated from vacuum.
>
> Index Scan using usertable_p_key on usertable (cost=0.00..25.68 rows=50
> width=72)
>
> That rows=50, which is an overestimate by the way.
That's because the estimate in this case was 50 and so it's estimating
that going through the index and checking the heap is faster than a
sequence scan. The *estimator* didn't use the index to figure that out,
it's just saying that the best plan to actually *run* the query uses
the index.
IIRC, There's something which is effectively :
estimated rows = <most common value's frequency>*<fraction>
I think fraction defaults to (is always?) 1/10 for the standard
index type. That's where the 50 comes from. And the frequency is
probably from the last vacuum analyze.