Performance system values: Parallel processing for queries and indexes
The Parallel processing for queries and indexes system value is also known as QQRYDEGREE. You can use this system value to specify whether to use parallel processing.
| Quick reference | |
|---|---|
| Location | From IBM® Navigator for i, select . Right-click on Performance and click Properties, then select the Database tab. |
| Special authority | All object (*ALLOBJ) and security administrator (*SECADM). |
| Default value | Do not allow parallel processing. |
| Changes take effect | Immediately. |
| Lockable | No. |
What can I do with this system value?
You can specify whether to allow parallel processing and specify the type of
parallel processing. Parallel processing allows different processors to run at the same time so that
the system can perform queries and build indexes faster. 
Activating parallel
processing system wide with the QQRYDEGREE system value is not recommended since queries processed
with parallel access methods aggressively use main storage, CPU, and disk resources. Parallel
processing is more appropriate to request for specific jobs through the use of the CURRENT DEGREE
special register or the PARALLEL_DEGREE QAQQINI option. Parallel processing is discussed at this
link: Controlling parallel processing
for queries
Types of parallel processing include input/output (I/O) parallel processing and symmetric multiprocessing (SMP). For I/O parallel processing, the database manager can use multiple tasks for I/O auxiliary storage processing for each query. Processing for the central processing unit (CPU) is still done serially. For SMP, the CPU and I/O processing are assigned to tasks that run the query in parallel. Actual CPU parallelism requires a system with multiple processors. SMP parallelism is used only if the system feature DB2® Symmetric Multiprocessing for i5/OS is installed.
This system value has the following options:
- Do not allow parallel processing (*NONE)
- No parallel processing is allowed for database query processing.
- Use multiple processes for input/output (*IO)
- The CQE optimizer can use parallel I/O methods only. This setting does not require the SMP feature. The SQE optimizer considers I/O parallelism with or without this setting.
- Use multiple processes for input/output, queries, and indexes (*OPTIMIZE)
- The query optimizer can use any number of tasks for either I/O or SMP parallel processing for queries. If you select to use multiple processes for input/output, queries, and indexes, you can also select to Use all active memory in memory pool (*MAX). Using all active memory allows the query optimizer to choose an access plan that might consume more of the system resources, but would provide faster run time for a given query.