How can I use Index on a DATE Field

Hi, All:
  Need Your Help!
  I have created an index on a table, but the index field which data type is DATE.
  I found if I execute the sql statement like that :

     select * from table where fdate > sysdate - 10;   oracle will use index searching,
but if I execute the sql statement:

    select * from table where to_char(fdate,'yyyymmdd') => '19991001'   oracle will use FULL table scan,
Can anyone let me know How to force Oracle to use INDEX SCAN ?

Thanks for your help
Allen Received on Wed Nov 17 1999 - 00:50:42 CST