On Friday, October 21, 2011 08:14:12 PM Robert Haas wrote: > On Fri, Oct 21, 2011 at 2:08 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Robert Haas <robertmh...@gmail.com> writes: > >> On Fri, Oct 21, 2011 at 1:18 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > >>> 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. > > > > It's not "touching six times less data". It's touching the exact same > > number of tuples either way, just index tuples in one case and heap > > tuples in the other. > > Yeah, but it works out to fewer pages. But access to those is not sequential. I guess if you measure cache hit ratios the index scan will come out significantly worse.
Andres -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers