Select Performance Using Order By on Index

Hi, I have a peformance problem when using select and order by on a multi field index that, hopefully, some of you SQL gurus can help me with. The problem is as follows: I need to select and fetch forwards in the natural index order where the index contains multiple segments on a single data table. To illustrate the point and for the sake of simplicity I will use numeric fields as follows: field1 field2 field3 ----------------------------- 1 5 1 1 5 2 1 6 0 1 6 1 1 6 2 1 6 3 2 4 3 The index I have created is unique and consists of all three fields in order. The query that I use is as follows: SELECT * FROM find WHERE find_field1 = 1 AND find_field2 = 5 AND find_field3 >= 1 OR find_field1 = 1 AND find_field2 > 5 OR find_field1 > 1 ORDER BY find_field1, find_field2, find_field3 Doing this query on small tables gives reasonable performance, however, if I do the query on large tables (approx 30000 records) and the required record is towards the end of the index the select becomes unacceptably slow. Note that the extent sizes on the table are correctly set up. Changing the query to the following: SELECT * FROM find WHERE find_field1 >= AND find_field2 >= 5 AND find_field3 >= 1 ORDER BY find_field1, find_field2, find_field3 provides intantaneous response (so I do know that informix can do it) yet provides an incorrect result set when scrolling forwards ie. the third and last record are no longer part of the result set. Comparing the cost using 'explain' also gives an entire different picture for the two queries ie. the first query is much more expensive than the second, yet I feel that the second should be more expensive as records in the natural index sequence have to be skipped. Note that in both cases informix reports that it does use the index. The questions I have are as follows: 1. Why is this the case on a naturally ordered index? 2. What can I do to improve the select performance with the result set being what I require? Thanks in advance, Theo Kramer theo@rascal.co.za