change logging

2 questions regarding IDS: 1) how can I change a database from no logging to unbuffered logging? When I use onmonitor it tells me to use ontape. If I use ontape it tells me that I'm trying an invalid logging change. 2) I used to work with Informix SE. I have switched to IDS and it is way slower. Especially index handling is not clear to me. I have a compound index on two attributes. If I make a select on the first, order by on the second it takes forever. If I order on the first and the second it's fast. How can I tell the optimizer to always use the compound index= Your help is very much appreciated. Benno