The selectivity of a condition is the fraction of the table’s rows that satisfy that condition.
The SAP Sybase IQ query optimizer uses information from available indexes to select an appropriate strategy for executing a query. For each condition in the query, the optimizer decides whether the condition can be executed using indexes, and if so, the optimizer chooses which index and in what order with respect to the other conditions on that table. The most important factor in these decisions is the selectivity of the condition; that is, the fraction of the table’s rows that satisfy that condition.
The optimizer normally decides without user intervention, and it generally makes optimal decisions. In some situations, however, the optimizer might not be able to accurately determine the selectivity of a condition before it has been executed. These situations normally occur only where either the condition is on a column with no appropriate index available, or where the condition involves some arithmetic or function expression and is, therefore, too complex for the optimizer to accurately estimate.
If you have a query that is run frequently, then you may want to experiment to see whether you can improve the performance of that query by supplying the optimizer with additional information to aid it in selecting the optimal execution strategy.