Re: Query using SeqScan instead of IndexScan

On 4/2/06, chris smith <dmagick(at)gmail(dot)com> wrote:
> On 4/2/06, Jim C. Nasby <jnasby(at)pervasive(dot)com> wrote:
> > On Sat, Apr 01, 2006 at 11:23:37AM +1000, chris smith wrote:
> > > On 4/1/06, Brendan Duddridge <brendan(at)clickspace(dot)com> wrote:
> > > > Hi Jim,
> > > >
> > > > I'm not quite sure what you mean by the correlation of category_id?
> > >
> > > It means how many distinct values does it have (at least that's my
> > > understanding of it ;) ).
> >
> > Your understanding is wrong. :) What you're discussing is n_distinct.

<rant>
It'd be nice if the database developers agreed on what terms meant.

http://dev.mysql.com/doc/refman/5.1/en/myisam-index-statistics.html

The SHOW INDEX statement displays a cardinality value based on N/S,
where N is the number of rows in the table and S is the average value
group size. That ratio yields an approximate number of value groups in
the table.
</rant>

A work colleague found that information a few weeks ago so that's
where my misunderstanding came from - if I'm reading that right they
use n_distinct as their "cardinality" basis.. then again I could be
reading that completely wrong too.

I believe postgres (because it's a lot more standards compliant).. but
sheesh - what a difference!

This week's task - stop reading mysql documentation.

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