On Sun, Jun 5, 2016 at 9:03 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Claudio Freire <klaussfre...@gmail.com> writes: >> So correlated index scans look extra favourable vs bitmap index scans >> because bitmap heap scans consider random page costs sans correlation >> effects (even though correlation applies to bitmap heap scans as >> well). > > Really? How? The index ordering has nothing to do with the order in > which heap tuples will be visited.
It is not the order itself, but the density. If the index is read in a range scan (as opposed to =ANY scan), and the index lead column is correlated with the table ordering, then the parts of the table that need to be visited will be much denser than if there were no correlation. But Claudio is saying that this is not being accounted for. Cheers, Jeff -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
