Informix SE vs. Online Query

I have this one application that is running on 2 machines. They are both sun sparc 2 and running Informix 4.1 . The difference is that one machine is using Informix Standard Engine (SE) and the other one is using Online Engine. The data schema is identical on both machines and the amount of data is the same too. I am researching to see why this application is taking a long time to do a query on the SE and it takes no time at all on the Online ( I did take into consideration that the Online should be faster but still the difference is very significant , like 1 minute versus 50 minutes). So I used 'set explain on' and here are the results: Query: Select count(*) from table1 where field1 not in (select field1 from table2) Note: field1 has a unique index on both table1 and table2 Results: On the SE, the subquery is done thru a SEQUENTIAL SCAN On the Online, the subquery is done thru a INDEX PATH Index keys: field1 (Key-Only) I have called Informix Technical Support and the engineer first told me that it was correct the way how the SE optimizer worked (doing SEQUENTIAL SCAN) and then when I told him about the way how Online optimized the query, he suggested me to check for index corruption in the SE ???? Anyway,I complied and found nothing wrong with the index on both machines . QUESTIONS: 1) Is the SE optimizer doing the correct thing ,i.e. doing a SEQUENTIAL SCAN? 2) If SE optimizer is doing the correct thing, then is it correct to say that the Online Optimizer is in a way 'enhanced' and knows how to make better use of the indexes? Any input would be very much appreciated. Thanks!