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 Thu Mar 14 2002 - 23:55:44 CST
