Query performance and optimization

Queries that reference partitioned tables need to be carefully considered because partitioned tables are often very large. It is important to understand the effects of accessing multiple partitions on your system and applications.

Partitioned tables can take advantage of all optimization and parallel processing available using SQL on DB2 Universal Database™ for iSeries™. For general information about query optimization, see the Database performance and optimization. For partitioned tables, all the data access methods described in the Database performance and optimization topic can be used to access the data in each partition. In addition, if the DB2^® UDB Symmetric Multiprocessing feature is installed, the parallel data access methods are available for the optimizer to consider when implementing the query.

If queries need to access only an individual partition in a partitioned table, creating an alias for that individual partition and then using that alias in the query can enhance performance. The query acts as a nonpartitioned table query.

Partitioned tables conceptually implemented as a nested table where each partition is unioned to the other partitions.

SELECT LASTNAME, SALARY FROM PRODLIB.PAYROLL
	WHERE SALARY > 20000

SELECT LASTNAME, SALARY FROM 
		(SELECT LASTNAME, SALARY FROM  PRODLB.PAYROLL(PART00001)
		 UNION ALL
  		SELECT LASTNAME, SALARY FROM  PRODLB.PAYROLL(PART00002)
		UNION ALL
		SELECT LASTNAME, SALARY FROM  PRODLB.PAYROLL(PART00003)) 	
	X (LASTNAME, SALARY)
	WHERE X.SALARY > 20000

The implementation of partitioned table queries depends on which query engine is used: the Classic Query Engine (CQE) or the SQL Query Engine (SQE). For more information about the query engines, see the SQE and CQE Engines article in the Database performance and optimization topic. There are different considerations for each engine.

• Queries using SQL Query Engine
  The SQL Query Engine (SQE) provides targeted optimization for partitioned tables using dynamic partition expansion optimization.
• Queries using Classic Query Engine
  When queries are implemented using Classic Query Engine (CQE), you need to be aware of how the query is optimized including materialization and index usage.