"John D. Burger" <john@mitre.org> writes:
> Tom Lane wrote:
>> IIRC I picked an equation out of the literature partially on the basis
>> of it being simple and fairly cheap to compute...
> I'm very curious about this - can you recall where you got this, or
> at least point me to where in the code this happens?
src/backend/commands/analyze.c, around line 1930 as of CVS HEAD:
/*----------
* Estimate the number of distinct values using the estimator
* proposed by Haas and Stokes in IBM Research Report RJ 10025:
* n*d / (n - f1 + f1*n/N)
* where f1 is the number of distinct values that occurred
* exactly once in our sample of n rows (from a total of N),
* and d is the total number of distinct values in the sample.
* This is their Duj1 estimator; the other estimators they
* recommend are considerably more complex, and are numerically
* very unstable when n is much smaller than N.
*
* Overwidth values are assumed to have been distinct.
*----------
*/
regards, tom lane
