insert from select (table scan) with no join

Hope this comes out correctly, I have a process that is inserting from a select statement with no join. The rows in the table (~70,000 rows) all meet the filter so all rows will be inserted into the table. Sometimes the insert takes a 30 seconds - 1 minuite, then there are times that it take 20 minutes. Does any one have ideas why this may happen? Looking at the insert in xtree shows the table being scanned but the rows read is very small and infrequent. Specs: 9.40 FC2 HPUX 11 6 way box 6 gig - (4.2 gig for db) OLTP 315 gig. sending to informix-list

sometimes an index exisist on the table and sometimes it doesn't?