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. > You've arranged the test case so that all these > tuples are in shared buffers already, so there's no data movement to be > avoided. What this test case proves is that btree's overhead per index > tuple touched is significantly more than the cost of the fastest path > through HeapTupleSatisfiesMVCC, which I don't find surprising > considering how much sweat has been expended on that code path over the > years. I think HeapTupleSatisfiesMVCC is probably being skipped anyway in this case, since all the heap pages should be PD_ALL_VISIBLE. > (It may well be that it's not even btree at fault but the per-tuple > visits to the visibility map ... did you do any oprofiling yet?) No, but I think that might be a good idea. Maybe I'll go do that. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
