The dreaded sequential scan

Informix 7.10.UC1 has a tendency to perform sequential scans (which are *SLOW*) even when an index is available. According to Informix, the way to avoid this problem is to run UPDATE STATISTICS after major changes are made to a table. My problem is that I'm working on a project which is supposed to store 20 million records using Informix. First, what happens when you run UPDATE STATISTICS on a live data base this size? Our system doesn't have any scheduled down time, so that we need to support queries (and probably updates) of the data base while UPDATE STATISTICS is running. Second, how do we know when to run UPDATE STATISTICS? Running UPDATE STATISTICS after the system starts performing sequential scans is not an acceptable long term solution; we have to avoid sequential scans in the first place. Kenneth Almquist