index is separate file attached to dataset.
create index on variable that is used in 'by processing' 
or 'where processing'.
it will enhance the performance and fast retrieving the 
observations when you use indexed variable in where 
statement.
you need not to sort the variable if it is indexed when u 
use it in "by statement".
create an index on variable when:
-u have to retrieve observations not more than 15% of d 
dataset.
-create index on variable where 'by-variable' will have 
more than 8 disctint values.

index is of two types:
simple index-created on single variable.
composite index-created on 2 or more variables.
index can b created using proc datasets, proc sql and 
datastep.