SET CURRENT QUERY OPTIMIZATION statement

The SET CURRENT QUERY OPTIMIZATION statement assigns a value to the CURRENT QUERY OPTIMIZATION special register. The value specifies the current class of optimization techniques enabled when preparing dynamic SQL statements. It is not under transaction control.

Invocation

This statement can be embedded in an application program or issued through the use of dynamic SQL statements. It is an executable statement that can be dynamically prepared.

Authorization

None required.

Syntax

                                      .-=-.   
>>-SET--CURRENT--QUERY--OPTIMIZATION--+---+--------------------->

>--+-0-------------+-------------------------------------------><
   +-1-------------+   
   +-2-------------+   
   +-3-------------+   
   +-5-------------+   
   +-7-------------+   
   +-9-------------+   
   '-host-variable-'   

Description

optimization-class

optimization-class can be specified either as an integer constant or as the name of a host variable that will contain the appropriate value at run time. An overview of the classes follows.

0

Specifies that a minimal amount of optimization is performed to generate an access plan. This class is most suitable for simple dynamic SQL access to well-indexed tables.

1

Specifies that optimization roughly comparable to DB2® Version 1 is performed to generate an access plan.

2

Specifies a level of optimization higher than that of DB2 Version 1, but at significantly less optimization cost than levels 3 and above, especially for very complex queries.

3

Specifies that a moderate amount of optimization is performed to generate an access plan.

5

Specifies a significant amount of optimization is performed to generate an access plan. For complex dynamic SQL queries, heuristic rules are used to limit the amount of time spent selecting an access plan. Where possible, queries will use materialized query tables instead of the underlying base tables.

7

Specifies a significant amount of optimization is performed to generate an access plan. Similar to 5 but without the heuristic rules.

9

Specifies a maximal amount of optimization is performed to generate an access plan. This can greatly expand the number of possible access plans that are evaluated. This class should be used to determine if a better access plan can be generated for very complex and very long-running queries using large tables. Explain and performance measurements can be used to verify that a better plan has been generated.

host-variable

The data type is INTEGER. The value must be in the range 0 to 9 (SQLSTATE 42815) but should be 0, 1, 2, 3, 5, 7, or 9. If host-variable has an associated indicator variable, the value of that indicator variable must not indicate a null value (SQLSTATE 42815).

Notes

• When the CURRENT QUERY OPTIMIZATION register is set to a particular value, a set of query rewrite rules are enabled, and certain optimization variables take on particular values. This class of optimization techniques is then used during preparation of dynamic SQL statements.
• In general, changing the optimization class impacts the execution time of the application, the compilation time, and resources required. Most statements will be adequately optimized using the default query optimization class. Lower query optimization classes, especially classes 1 and 2, may be appropriate for dynamic SQL statements for which the resources consumed by the dynamic PREPARE are a significant portion of those required to execute the query. Higher optimization classes should be chosen only after considering the additional resources that may be consumed and verifying that a better access plan has been generated.
• Query optimization classes must be in the range 0 to 9. Classes outside this range will return an error (SQLSTATE 42815). Unsupported classes within this range will return a warning (SQLSTATE 01608) and will be replaced with the next lowest query optimization class. For example, a query optimization class of 6 will be replaced by 5.
• Dynamically prepared statements use the class of optimization that was set by the most recently executed SET CURRENT QUERY OPTIMIZATION statement. In cases where a SET CURRENT QUERY OPTIMIZATION statement has not yet been executed, the query optimization class is determined by the value of the dft_queryopt database configuration parameter.
• Statically bound statements do not use the CURRENT QUERY OPTIMIZATION special register; therefore this statement has no effect on them. The QUERYOPT option is used during preprocessing or binding to specify the desired class of optimization for statically bound statements. If QUERYOPT is not specified then, the default value specified by the dft_queryopt database configuration parameter is used.
• The results of executing the SET CURRENT QUERY OPTIMIZATION statement are not rolled back if the unit of work in which it is executed is rolled back.

Examples

   SET CURRENT QUERY OPTIMIZATION 9

Example 2:  The following example shows how the CURRENT QUERY OPTIMIZATION special register can be used within a query.

   EXEC SQL DECLARE C1 CURSOR FOR
     SELECT PKGNAME, PKGSCHEMA FROM SYSCAT.PACKAGES
     WHERE QUERYOPT = CURRENT QUERY OPTIMIZATION