Most seasoned SQL programmers know that when you use host variables, DB2 may not always come up with the absolutely most optimal access path at bind time. Without knowing the actual values of the host variables the optimizer has to make some best guesses as to how best to satisfy the SQL request.
Furthermore, we know that we can guide DB2 on how best to approach this situation using the REOPT parameter of the BIND command. Prior to DB2 V9, there were three options for REOPT:
REOPT(NONE) – DB2 will not reoptimize SQL at run time.
REOPT(ALWAYS) – DB2 will prepare SQL statements again at run time when the host variable values are known. This enables the DB2 optimizer to formulate the query execution plan using the actual host variable values, which can result in better performing access paths.
REOPT(ONCE) – DB2 will prepare SQL statements only once, using the first set of host variable values, no matter how many times the statement is executed by the program. The access path is stored in the Dynamic Statement Cache (DSC) and will be used for all subsequent executions of the same SQL statement. REOPT(ONCE) only applies to dynamic SQL statements and is ignored if you use it with static SQL statements. This option was introduced in DB2 V8.
What is New in V9?
DB2 9 for z/OS introduces a new REOPT option: REOPT(AUTO). The ideas behind REOPT(AUTO) is to come up with the optimal access path in the minimum number of prepares.
The basic premise of REOPT(AUTO) is to re-optimize only when host variable values change. Using this option, DB2 will examine the host variable values and will generate new access paths only when host variable values change and DB2 has not already generated an access path for those values.
REOPT(AUTO) only applies to dynamic statements that can be cached.
After migrating to DB2 9, consider re-evaluating programs bound specifying REOPT(ALWAYS) and REOPT(NONE). In many cases, switching to REOPT(AUTO) from REOPT(ALWAYS) can produce performance improvement; and in some cases you can use re-optimization with REOPT(AUTO) for programs bound REOPT(NONE) because of the fear of too frequent re-optimization causing a performance hit.
In particular, consider specifying REOPT(AUTO) for SQL statements that at times can take a relatively long time to execute, depending on the values of parameter markers. In particular, you should especially consider doing this when parameter markers refer to non-uniform data that is joined to other tables.