Re: More thoughts about planner's cost estimates

Greg,

> Using a variety of synthetic and real-world data sets, we show that
> distinct sampling gives estimates for distinct values queries that
> are within 0%-10%, whereas previous methods were typically 50%-250% off,
> across the spectrum of data sets and queries studied.

Aha. It's a question of the level of error permissable. For our
estimates, being 100% off is actually OK. That's why I was looking at 5%
block sampling; it stays within the range of +/- 50% n-distinct in 95% of
cases.

> Doing a bit of basic searching around I think the tool we're looking for
> here is called a "chi-squared test for independence".

Augh. I wrote a program (in Pascal) to do this back in 1988. Now I can't
remember the math. For a two-column test it's relatively
computation-light, though, as I recall ... but I don't remember standard
chi square works with a random sample.

--
--Josh

Josh Berkus
PostgreSQL @ Sun
San Francisco