Query Optimising Problem !!

I have an application running on Online 5.01. Recently the performance has been degraded, I have checked with Informix Support & they verified the database is actually quite well tuned. So, they propose us to do some study on the query execution plan to optimise the transaction. I have never done any query optimisation practise in Informix Online before, can anyone provide me some pointers ?? Any good reference book ?? Any idea will be appreciated. Thanks in Advance. Alan Alan Chen ------------------------------------------------------------------------------- SGS-Thomson Microelectronics, | Phone : 4821411 Ext 5335 28 Ang Mo Kio, Industrial Park II, | Fax : (65) 4820240 Singapore 2056. | Internet : sgs@technet.sg -------------------------------------------------------------------------------