The SQL optimizer can only optimize data access for SQL statements that contain a search condition. Search conditions can occur in the following situations:
in a WHERE clause
in a join which is defined using ANSI syntax
SELECT name FROM customer INNER JOIN ORDER ON customer.ID = order.ID and order.ID = 4711 WHERE customer.name = 'CUSTOMER'
You can specify search conditions for the following types of SQL statements:
Note that restrictions apply, see Restrictions for UPDATE Statements.
The form of the search condition has a significant impact on the costs incurred by executing the SQL statement. The SQL optimizer only takes into account the following search conditions:
If the SQL optimizer cannot transform the search conditions in an SQL statement into one of the forms listed above, it cannot determine a search strategy for them and uses the sequential search instead. If search conditions are linked using several identical boolean operators, the sequence of these conditions does not affect how the SQL optimizer determines the search strategy.
SQL Reference Manual, Table Expression (table_expression)