Here is the link to the Oracle documentation page which says:

Path 4: Single Row by Unique or Primary Key
This access path is available if the statement's WHERE clause uses all columns of a unique or primary key in equality conditions. For composite keys, the equality conditions must be combined with AND operators. To execute the statement, Oracle performs a unique scan on the index on the unique or primary key to retrieve a single rowid, and then accesses the table by that rowid.

So optimizer does not use unique key constraint (index) if all the columns are not in where clause.
Here is the link:
[url]http://technet.oracle.com/docs/products/oracle8i/doc_index.htm[/url]