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
