I had read it somewhere but didn't bookmark the link , But As far as I remember I put here right thing
If there is composite index on a table then first column of index must be required in query to use the index .
Therse are few more thing I want to add
When two columns are not unique individually but are unique together, composite indexes might work very well. For example, although columns A and B have few unique values, rows with a particular combination of columns A and B are mostly unique. Look for WHERE clauses with AND operators.
If all values of a SELECT statement are in a composite index, Oracle does not query the table; the result is returned from the index