Re: count * performance issue

A Dimarts 11 Març 2008 04:11, Scott Marlowe va escriure:
> On Mon, Mar 10, 2008 at 7:57 PM, Robins Tharakan <tharakan(at)gmail(dot)com> wrote:
> > Hi,
> >
> > I have been reading this conversation for a few days now and I just
> > wanted to ask this. From the release notes, one of the new additions in
> > 8.3 is (Allow col IS NULL to use an index (Teodor)).
> >
> > Sorry, if I am missing something here, but shouldn't something like this
> > allow us to get a (fast) accurate count ?
> >
> > SELECT COUNT(*) from table WHERE indexed_field IS NULL
> > +
> > SELECT COUNT(*) from table WHERE indexed_field IS NOT NULL
>
> It really depends on the distribution of the null / not nulls in the
> table. If it's 50/50 there's no advantage to using the index, as you
> still have to check visibility info in the table itself.
>
> OTOH, if NULL (or converserly not null) are rare, then yes, the index
> can help. I.e. if 1% of the tuples are null, the select count(*) from
> table where field is null can use the index efficiently.

But you'll get a sequential scan with the NOT NULL case which will end up
taking more time. (Seq Scan + Index Scan > Seq Scan)

--
Albert Cervera Areny
Dept. Informàtica Sedifa, S.L.

Av. Can Bordoll, 149
08202 - Sabadell (Barcelona)
Tel. 93 715 51 11
Fax. 93 715 51 12

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