To ensure that the cost-based database optimizer functions properly, the table and index statistics must be updated regularly. The database optimizer uses these statistics as a basis for selecting the access strategy used to execute a particular SQL statement.
The database administrator must update the table and index statistics regularly. To find out when the last update was performed, from the SAP standard menu choose Tools >> CCMS >> DB administration >> DBA Planning Calendar, or use Transaction DB13. You must ensure that the last update was not performed more than a week ago, and that it was performed successfully (refer to the log).
For making changes to index design, we recommend that roles and responsibilities are distributed as follows. The ABAP programmer designs the database indexes to support SQL statements. In a production system or test system, only the database administrator should be responsible for creating database indexes. The database administrator must verify whether the programmer's index design is correct.
When database indexes are created, the table and index statistics must be updated for the relevant tables. If the index design is correct, you can transport the database index to the production system at a time of low system activity, and then update the table and index statistics in the production system.
The way in which the database optimizer selects and uses database indexes depends on, among other things, the WHERE conditions specified in the SQL statement. The order of the WHERE conditions in the SQL statement is not important (note that this is not the case with the order of the fields in the database index).
The index search string is concatenated over the fields that are specified without gaps and with '=' in the WHERE condition. The index blocks to be checked are are selected using the index search string.
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