On Mon, Oct 10, 2011 at 3:15 PM, Kevin Grittner <kevin.gritt...@wicourts.gov> wrote: > Robert Haas <robertmh...@gmail.com> wrote: > >> Right now, our costing model for index-only scans is pretty dumb. >> It assumes that using an index-only scan will avoid 10% of the >> heap fetches. That could easily be low, and on an insert-only >> table or one where only the recently-updated rows are routinely >> accessed, it could also be high. > > As a reality check, I just ran this query on a table in a statewide > copy of our data: > > select count(*), > sum(case when xmin = '2'::xid then 0 else 1 end) as read_heap > from "CaseHist"; > > and got: > > count | read_heap > -----------+----------- > 205765311 | 3934924 > > So on our real-world database, it would skip something on the order > of 98% of the heap reads, right?
Yeah, if it's scanning the whole table. -- 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
