"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. regards, tom lane -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers