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 - 08:43:31 CST
