On Fri, May 11, 2012 at 11:04 AM, Kevin Grittner
<Kevin.Grittner@wicourts.gov> wrote:
> We
> will be probing random page numbers 1..P for random tuple indexes
> 1..M. So how many random probes by ctid does that require? The
> chance of a hit on each probe is ((T/P)/M) -- the average number of
> tuples per page divided by the number of tuple index values allowed.
> So we need (S*T)/((T/P)/M) probes. Simplifying that winds up being
> S*M*P the product of the sample size as a percentage, the maximum
> tuple index on a page, and the number of pages. (A calculation some
> may have jumped to as intuitively obvious.)
>
> So let's call the number of probes N. We randomly generate N
> distinct ctid values, where each is a random page number 1..P and a
> random index 1..M. We attempt to read each of these in block number
> order, not following HOT chains. For each, if the tuple exists and
> is visible, it is part of our result set.
>
> Since T cancels out of that equation, we don't need to worry about
> estimating it. Results will be correct for any value of M which is
> *at least* as large as the maximum tuple index in the table,
> although values of M larger than that will degrade performance. The
> same holds true for the number of pages.
The trouble is, AFAICS, that you can't bound M very well without
scanning the whole table. I mean, it's bounded by theoretical limit,
but that's it.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
