cardinality= measure of selectivity
the cardinality of sex='male' in the clubmembers table of a club that only
allows men is low.(highly non-selective)
Jim
"Ryan Gaffuri" <rkg100_at_erols.com> wrote in message
news:6757365f.0203150643.2857cbdf_at_posting.google.com...
> What do you mean by selective? Does this have to do with the
> cardinality of the data?
>
> "Jim Kennedy" <kennedy-family_at_attbi.com> wrote in message
news:<A1gk8.41173$q2.5851_at_sccrnsc01>...
> > the where clause is the predicate.
> > eg
> > select * from emp where sex='male';
> > sex= 'male' is the predicate.
> > If the statistics are up to date on the table and indexes on this table
it
> > will have an idea how selective the index on sex (assuming it exists)
is.
> > Histograms record the distribution of the data.
> > Jim
> > "Ryan Gaffuri" <rkg100_at_erols.com> wrote in message
> > news:a6rqlt$q8n$1_at_bob.news.rcn.net...
> > > Ive been reading the OTN performance tuning guide. I dont really
> > understand
> > > this paragraph.
> > >
> > > What is meant by predicate?
> > >
> > > "It is possible for the DBA to generate statistics that quantify the
data
> > > distribution and storage characteristics of tables, columns, indexes,
and
> > > partitions. The cost-based optimization approach uses these statistics
to
> > > calculate the selectivity of predicates and to estimate the cost of
each
> > > execution plan. Selectivity is the fraction of rows in a table that
the
> > SQL
> > > statement's predicate chooses. The optimizer uses the selectivity of a
> > > predicate to estimate the cost of a particular access method and to
> > > determine the optimal join order."
> > >
> > >
Received on Fri Mar 15 2002 - 09:10:34 CST
