On Fri, Oct 21, 2011 at 1:18 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> That's a bit disappointing - it's now more than a third faster to do >> the sequential scan, even though the sequential scan has to touch six >> times as many blocks (at scale factor 20, index is 43 MB, table is 256 >> MB) all of which are in cache. Of course, touching that many fewer >> blocks does have some advantages if there is concurrent activity on >> the system, but it still seems unfortunate that the ratio of runtime >> to blocks touched is more than 8x higher for the index-only case. > > I don't know why you'd imagine that touching an index is free, or even > cheap, CPU-wise. The whole point of the index-only optimization is to > avoid I/O. When you try it on a case where there's no I/O to be saved, > *and* no shared-buffers contention to be avoided, there's no way it's > going to be a win.
Well, call me naive, but I would have thought touching six times less data would make the operation run faster, not slower. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers