On Thu, Jul 26, 2012 at 12:17 PM, Robert Haas <robertmh...@gmail.com> wrote: >> I think so. The case where you want a wide multiple column primary >> key to be extended to cover that one extra commonly grabbed value is >> not super common but entirely plausible. With the existing >> infrastructure to get the advantages of index covering you'd have to >> duplicate the entire index for the extra field which really sucks: >> aside from the huge waste of time and space, you force the planner to >> play the 'which index do i use?' game. > > I think it is going to take several years before we really understand > how index-only scans play out in the real world, and what factors > limit their usefulness. This one has come up a few times because it's > sort of an obvious thing to want to do and we don't have it, but I > think that there's room for some skepticism about how well it will > actually work, for reasons that have already been mentioned, and of > course also because indexing more columns potentially means defeating > HOT, which I suspect will defeat many otherwise-promising applications > of index-only scans.
Sure. many will still get to use them though: I'm doing tons of OLAP/BI lately: wide keys, minimal to no updating, minimal to no RI, andvery large tables (often clustered and partitioned), and extreme performance requirements. Covering indexes to me is basically a drop in feature and COVERING seems to make a lot of sense on paper. (I absolutely can't wait to get 9.2 on some of our bigger servers here). merlin -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers