
Bruce Momjian wrote:
> > A different approach that's been discussed on pghackers is to make use
> > of btree indexes for columns that have such indexes: we could scan the
> > indexes to visit all the column values in sorted order.  I have rejected
> > that approach because (a) it doesn't help for columns without a suitable
> > index; (b) our indexes don't distinguish deleted and live tuples,
> > which would skew the statistics --- in particular, we couldn't tell a
> > frequently-updated single tuple from a commonly repeated value; (c)
> > scanning multiple indexes would likely require more total I/O than just
> > grabbing sample tuples from the main table --- especially if we have to
> > do that anyway to handle columns without indexes.
>
> Remember one idea is for index scans to automatically update the expired
> flag in the index bitfields when they check the heap tuple.

    And  we  should  really do that. While playing around with my
    (for 7.2 to be) access statistics stuff  I  found  that  when
    running  pg_bench,  a  couple  of  thousand index scans cause
    millions  and  millions  of  buffer  fetches,  because   that
    pg_bench updates one and the same row over and over again and
    it has a PKEY.


Jan

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Mon, 23 Apr 2001 13:15:02 -0500 (EST)
Jan Wieck <JanWieck@Yahoo.com>
Re: RFC: planner statistics in 7.2