OWS performance problem

Hi I have an Informix Online Workgroup server 7.20.UC2 on SCO Unix 5.0.2 After first install, it was dead slow. I did an update statistics high and it worked much quicker. One problem stays : if I have a composed index (A,B) and i do a select "where A="X" and B matches "*Y*" it takes minutes before i have an answer. If I drop the index, it is much quicker. sqexlain.out says that with the index, OWS uses the index, without, he does a seq.scan. I changed OPTCOMPIND to 0, I did an update statistics high on A and on B without result. HELP ! Guy