index scans

range scan :

when the indexed column is  referenced in the where clause .

index fast full scan :

when the column s in the select clause is all indexed

index full scan :

it scans the index blocks fully in the unsorted manner

index skip scan :

this is used when there is a composit index and the second column is referenced .

eg: cretae table t (a number,b number,c number);

create index a_idx on t(a);
create index b_idx on t(b);

select * from t where b=12 (index skip scan)

select * form t wher ea=10 and b=12 (range scan)

select a,b from t (index fast full scan or full scan )