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Re: Subquery flattening causing sequential scan

From: Ondrej Ivanič <ondrej(dot)ivanic(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Subquery flattening causing sequential scan
Date: 2011-12-27 22:21:00
Message-ID: CAM6mieL3XY25gGQacD7EYnWg9z-P2=kAEN_15xAQvic=LQTa7w@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Hi,

On 28 December 2011 05:12, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Possibly raising the stats target on emsg_messages would help.

In the function std_typanalyze() is this comment:

       /*--------------------
         * The following choice of minrows is based on the paper
         * "Random sampling for histogram construction: how much is enough?"
         * by Surajit Chaudhuri, Rajeev Motwani and Vivek Narasayya, in
         * Proceedings of ACM SIGMOD International Conference on Management
         * of Data, 1998, Pages 436-447.  Their Corollary 1 to Theorem 5
         * says that for table size n, histogram size k, maximum relative
         * error in bin size f, and error probability gamma, the minimum
         * random sample size is
         *      r = 4 * k * ln(2*n/gamma) / f^2
         * Taking f = 0.5, gamma = 0.01, n = 10^6 rows, we obtain
         *      r = 305.82 * k
         * Note that because of the log function, the dependence on n is
         * quite weak; even at n = 10^12, a 300*k sample gives <= 0.66
         * bin size error with probability 0.99.  So there's no real need to
         * scale for n, which is a good thing because we don't necessarily
         * know it at this point.
         *--------------------
         */

The question is why the parameter f is not exposed as a GUC? Sometimes
it could make sense to have few bins with better estimation (for same
r).

-- 
Ondrej Ivanic
(ondrej(dot)ivanic(at)gmail(dot)com)

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