On Saturday, October 22, 2011 05:20:26 PM Tom Lane wrote: > Andres Freund <and...@anarazel.de> writes: > > 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: > >>> 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. > > Huh? In the case he's complaining about, the index is all in RAM. > Sequentiality of access is not an issue (at least not at the page > level --- within a page I suppose there could be cache-line effects). I was talking about L2/L3 caches...
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