Re: Improving N-Distinct estimation by ANALYZE

On Fri, Jan 13, 2006 at 11:37:38PM -0500, Tom Lane wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
> >> It's also worth mentioning that for datatypes that only have an "="
> >> operator the performance of compute_minimal_stats is O(N^2) when values
> >> are unique, so increasing sample size is a very bad idea in that case.
>
> > Hmmm ... does ANALYZE check for UNIQUE constraints?
>
> Our only implementation of UNIQUE constraints is btree indexes, which
> require more than an "=" operator, so this seems irrelevant.

IIRC, the point was that if we know a field has to be unique, there's no
sense in doing that part of the analysis on it; you'd only care about
correlation.
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Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
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