On Wed, Apr 8, 2009 at 10:33 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Wed, Apr 8, 2009 at 1:17 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> And please note that we think bitmap scans are the larger part of >>> the win anyway. What's left undone there is some marginal mopup. > >> Can you elaborate on this? I'm fuzzy on why index scans can't benefit >> from this as much as bitmap index scans. > > The main point is that the planner will prefer a bitmap scan for any > query that's estimated to return more than quite a small number of rows. > (In my experience the cutover point is in the single digits.) So > there's just not that much room to win for plain indexscans. Their > principal application is really for fetching single rows, a case where > prefetch is entirely useless because you have nothing to overlap.
That makes sense, but what about the nestloop-over-inner-indexscan case? ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
