sysptprof: bufreads, pagreads and isreads ?

Dear Informix Community, Who can help me to get a clearer picture about the figures in the sysptprof table (also provided by onstat -p)? What is the relationship between the figures for the ISAM calls and the figures for the Buffer Reads/Writes? How can it happen that the number of bufreads goes down but the number of isreads goes up when a different index is used by a query? How is it possible that I sometimes have iswrites but no bufwrites and pagwrites? (I am the only one using the server) Is it a good approach to try to minimize the bufreads (knowing all data is in cache) when tuning a query? Or should I focus more on the ISAM calls? Looking forward to hearing from you (and to see hopefully more clearer). Cheers, Georg