This is mainly useful for the fast retrival of data (For ex 
we have 559 in 1 lac obs then by index we can get the data 
with out reading all the observations

Index are 2 type:

1.simple
2.Composite


ex-for simple


proc datasets lib=college;
modify time;
index create class;
run;


ex-for composite:


proc datasets library=college;
modify time;
index create names(=(name school);
run;