Index for Informix Dynamic Server

Hello, We alreay upgrade our database server from SE5.X to IDS 7.3. After we convert all the old data to the new server, we have some problems on the index search. For example, a table: table : ith ( ithitno, ithname ), total 1,500,000 records index : ith_1 on ithitno, ithname when I execute the following SQL statement : SELECT * FROM ith WHERE ithitno="ABC" ( "ABC" is not a record in ith ) it take a very long time ( few minutes ) for the SQL to return "no record found". At the end we find that the SQL statement only use a sequential search to find the record "ABC". How can I control the use of the index on IDS. Please help me. Thanks