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

> Tom Lane wrote:
> > 
>  It turns out that the selectivity estimate for an "=" comparison is
> just
> > the attdisbursion statistic calculated by VACUUM ANALYZE, which can be
> > roughly defined as the frequency of the most common value in the column.
> > (I took statistics too long ago to recall the exact definition.)
> > Anyway, given that the test data Ole sent me contains nearly all ''
> > entries, I'd say that the 0.995 value is about right for disbursion.
> > 
> > Indeed, if one were to do a "select * from av_parts where nsn = ''",
> > then sequential scan would be the most efficient way to do that.
> > The system has no clue that that's not really something you'd do much.
> 
> Does the system currently index NULLs as well ?
> 
> I suspect supporting partial indexes (initially just non-NULLs) would 
> let us have much better and also use indexes intelligently for
> mostly-NULL 
> columns.
> 
> Perhaps a line like 
> 
> * Add partial index support
> 
> would fit in TODO
> 
> -----------------
> Hannu
> 
> 


Yes, I think we index nulls.  What are partial indexes?

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