On Tue, May 10, 2011 at 12:53 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > "Kevin Grittner" <kevin.gritt...@wicourts.gov> writes: >> Simon Riggs <si...@2ndquadrant.com> wrote: >>> This topic has been discussed many times, yet I have never seen an >>> assessment that explains WHY we would want to do index-only scans. > >> In databases with this feature, it's not too unusual for a query >> which uses just an index to run one or more orders of magnitude >> faster than a query which has to randomly access the heap for each >> index entry. That seems like enough evidence of its possible value >> in PostgreSQL to proceed to the point where benchmarks become >> possible. I'm assuming that, like all other features added as >> performance optimizations, it won't be committed until there are >> benchmarks showing the net benefit. > >> As a thought experiment, picture the relative costs of scanning a >> portion of an index in index sequence, and being done, versus >> scanning a portion of an index in index sequence and jumping to a >> random heap access for each index entry as you go. > > It's already the case that we'll flip over to a bitmap indexscan, > and thus get rid of most/all of the "random" page accesses, in > situations where this is likely to be a big win. Pointing to the > performance difference in databases that don't do that is therefore > not too convincing. > > I'm inclined to agree that index-only scans might be worth the amount > of work that's involved ... but I share Simon's desire to see some proof > before anything gets committed.
Well, we're not in the habit of committing performance patches without performance numbers, so I doubt we'll reverse that trend now, and certainly I had no intention of doing so. -- 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