On Mon, Oct 24, 2011 at 2:15 PM, Kevin Grittner <kevin.gritt...@wicourts.gov> wrote: >> It doesn't look to me like the mean is above 2 (unless you have >> many fewer toast tables than I suspect), so trying to optimize >> many-column cases isn't going to help. > > The mean is 2.4 (give or take a little depending on whether you > include system tables). I have no idea where the optimization > becomes worthwhile, but the assertion that most indexes probably > have a single column worried me. I'm sure there are databases where > that is true (especially for those who insist on adding a > meaningless surrogate key column to every table), but there are many > where it isn't true. I would guess that our average of 2.4 is > higher than most, though.
Keep in mind that the existence of index-only scans is going to encourage people to try to create covering indexes on columns that aren't indexed today. It's not clear how much of a win that will be; for certainly workloads, HOT might make it backfire spectacularly. But even though Tom's statement that most indexes are one column might be a slight exaggeration, I suspect it probably is true that the optimizations he's talking about for large numbers of columns won't produce any material benefit even for a 3 or 4 column index. Which makes me think maybe we should focus our efforts elsewhere. -- 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