On Wed, Oct 12, 2011 at 3:29 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> What I suggest as a first cut for that is: simply derate the visibility >>> fraction as the fraction >>> of the table expected to be scanned gets smaller. > >> I think there's a statistically more rigorous way of accomplishing the >> same thing. If you treat the pages we estimate we're going to read as >> a random sample of the population of pages then your expected value is >> the fraction of the overall population that is all-visible but your >> 95th percentile confidence interval will be, uh, a simple formula we >> can compute but I don't recall off-hand. > > The problem is precisely that the pages a query is going to read are > likely to *not* be a random sample, but to be correlated with > recently-dirtied pages.
Sure, but I was suggesting aiming for the nth percentile rather than a linear factor which I don't know has any concrete meaning. -- greg -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
