Hi, Dean! On Fri, Mar 18, 2016 at 1:20 PM, Dean Rasheed <dean.a.rash...@gmail.com> wrote:

> Probably a better URL to give is > http://www.adellera.it/investigations/distinct_balls/ which has a link > to the PDF version of the paper and also some supporting material. > > However, while that paper is in general very clear, I don't think it > gives a very clear explanation of that particular formula, and it > doesn't explain what it represents. It merely says that if "i" can be > ignored "for some reason (e.g. i << Nr)", then that formula is an > approximation to the exact "without replacement" formula, which is the > subject of that paper. > > But actually, that formula is the exact formula for the expected > number of distinct values when selecting *with replacement* from a > collection where each of the distinct values occurs an equal number of > times. So I think we should say that. > > Perhaps something along the lines of: > > /* > * Update the estimate based on the restriction selectivity. > * > * This uses the formula for the expected number of distinct > values > * when selecting with replacement from a collection where > each of > * the distinct values occurs an equal number of times (a > uniform > * distribution of values). This is a very close approximation > to > * the more correct method of selecting without replacement > when > * the number of distinct values is quite large --- for > example, > * see http://www.adellera.it/investigations/distinct_balls/. > * Additionally, it also turns out to work very well even when > the > * number of distinct values is small. > */ > +1 Thank you for work on this patch. The formula you propose and explanation look great! ------ Alexander Korotkov Postgres Professional: http://www.postgrespro.com The Russian Postgres Company