ASA SQL User's Guide
Query Optimization and Execution
Indexes
Adaptive Server Anywhere uses indexes to achieve other performance benefits. Having an index allows Adaptive Server Anywhere to enforce column uniqueness, to reduce the number of rows and pages that must be locked, and to better estimate the selectivity of a predicate.
Enforce column uniqueness Without an index, Adaptive Server Anywhere has to scan the entire table every time that a value is inserted to ensure that it is unique. For this reason, Adaptive Server Anywhere automatically builds an index on every column with a uniqueness constraint.
Reduce locks Indexes reduce the number of rows and pages that must be locked during inserts, updates, and deletes. This reduction is a result of the ordering that indexes impose on a table.
For more information on indexes and locking, see How locking works.
Estimate selectivity Because an index is ordered, the optimizer can estimate the percentage of values that satisfy a given query by scanning the upper levels of the index. This action is called a partial index scan.