Re: limit clause breaks query planner?

"Matt Smiley" <mss(at)rentrak(dot)com> writes:
> So an Index Scan is always going to have a higher cost estimate than
> an equivalent Seq Scan returning the same result rows (unless
> random_page_cost is < 1). That's why I think the planner is always
> preferring the plan that uses a Seq Scan.

If that were the case, we'd never choose an indexscan at all...

It's true that a plain indexscan is not preferred for queries that will
return a large fraction of the table. However, it should be willing to
use a bitmap scan for this query, given default cost settings (the
default cost settings will cause it to prefer bitmap scan for retrieving
up to about a third of the table, in my experience). I too am confused
about why it doesn't prefer that choice in the OP's example. It would
be interesting to alter the random_page_cost setting and see if he gets
different results.

regards, tom lane