How can optimize already existing index organized table?

Dear all,
I have IOT having 7 million records in oracle database, eventually iot use for fast access primary key but in my case, when i select primary key column it takes 5-4 seconds for select single column. My query is:<br>


Select Emp_Refno from Emp_master where Rownum =1 order by Emp_Refno asc;

I have also used Sql Tunning Advisor for optimize it and also get index suggest ion from SQL Tunning Advisor and also applied it, But in explain plan not seen this index and it takes same time after it.<br>