Sybase IQ supports a hint string that lets you specify per-predicate hints, such as selectivity, usefulness, index preference, and execution mode.
You can set selectivity in combination with three other query optimizations:
Setting the equivalent of an index preference option
Setting the usefulness (ordering the predicates)
Delaying one or more predicates
Under normal circumstances, there are no advantages to delaying evaluation, which could slow the query. If you choose to, however, you can move any of the following four behaviors to later in the query:
Before optimization
At first “first fetch” time
At second “first fetch” time (inside correlated subqueries or on the left side of a nested loop pushdown join only)
Not using indexes at all (“horizontal processing”)
For syntax, parameters, and examples, see “User-supplied condition hint strings,” Chapter 3, “SQL Language Elements,” in the Sybase IQ Reference Manual.
