ASA SQL User's Guide
  Query Optimization and Execution
    How the optimizer works

Automatic performance tuning

One of the most common constraints in a query is equality with a column value. The following example tests for equality of the sex column.

SELECT *
FROM employee
WHERE sex = 'f'

Queries often optimize differently at the second execution. For the above type of constraint, Adaptive Server Anywhere learns from experience, automatically allowing for columns that have an unusual distribution of values. The database stores this information permanently unless you explicitly delete it using the DROP STATISTICS command. Note that subsequent queries with predicates over that column may cause the server to recreate a histogram on the column.