From: | Hannu Krosing <hannu(at)trust(dot)ee> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Ole Gjerde <gjerde(at)icebox(dot)org>, pgsql-hackers(at)postgreSQL(dot)org |
Subject: | Re: Selectivity of "=" (Re: [HACKERS] Index not used on simple select) |
Date: | 1999-07-28 00:13:53 |
Message-ID: | 379E4B40.A9EF5A06@trust.ee |
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Lists: | pgsql-hackers |
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
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