ASA SQL User's Guide
Query Optimization and Execution
How the optimizer works
A number of assumptions underlie the design direction and philosophy of the Adaptive Server Anywhere query optimizer. You can improve the quality or performance of your own applications through an understanding of the optimizer's decisions. These assumptions provide a context in which you may understand the information contained in the remaining sections.
Minimal administration work
Optimize for first rows or for entire result set
Statistics are present and correct
An index can usually be found to satisfy a predicate
Virtual Memory is a scarce resource
Rewriting subqueries as EXISTS predicates