Re: Selectivity of "=" (Re: [HACKERS] Index not used on simple se lect)

> 
> > So, the selectivity that a search for the most common value would
> > have is a reasonable estimate for the selectivity of a search for any
> > value.  That's a bogus assumption in this case --- but it's hard to
> > justify making any other assumption in general.
> > 
> Other db's usually use the value count(*) / nunique for the light weight
> statistics.
> This makes the assumptoin that the distinct index values are evenly
> distributed.
> That is on average a correct assumption, whereas our assumption on average
> overestimates the number of rows returned.
> I am not sure we have a nunique info though.
> 

Yes, that's the problem.  Figuring out the number of uniques is hard,
expecially with no index.

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