PDQ Parameters

The PDQPRIORITY determines whether a query will be treated as a parallel query, it is a PDQ then this parameter quantifies the degree of parallelism that will be used. The PDQPRIORITY is a numeric value between 0 and 100, 0 being OFF and 100 being HIGH. The higher the number the greater the degree of parallelism that will be applied, and the greater the amount of memory and CPU that will be utilised. The degree of parallelisation can be determined as follows

#_secondary PDQ threads = (PDQPRIORITY / 100) * #CPUVP
#_scan_threads = #fragments_scanned_for_all_tables

PDQPRIORITY can be set in three places

	The ONCONFIG file.
	Environment variable. This overrides the configuration paremeter
	SET PDQPRIORITY statement in SQL. This overrides the environment variable

MAX_PDQPRIORITY limits the maximum PDQPRIORITY a session can run with. This is the absolute limit and can not be overridden not be increased once the engine has started. It can be viewed as the maximum degree of parallelism that the DBA will allow for the instance

DS_MAXQUERIES determines the quantum size of shared memory that will be allocated to a parallel database query. [BOOKMARK]. In addition to this it also limits the maximum number of parallel database queries that can run concurrently. If a query does not use decision support memory then it is not included in this parameter.

The number of active parallel database queries cannot exceed DS_MAXQUERIES.

DS_TOTAL_MEMORY is the total amount of shared memory [kbytes] available for parallel database queries.

The maximum number of scan threads that can be initialised is limited by DS_MAX_SCANS. A scan thread will be initialised for every fragement that is to be examined, therefore this number of threads can be large for highly fragmented tables.

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Some of these parameters can be changed using the onmode utility.

	DS_TOTAL_MEMORY	onmode -M
	DS_MAX_QUERIES	onmode -Q
	MAX_PDQPRIORITY	onmode -D
	DS_MAX_SCANS	onmode -S

If there are aleady parallel queries running then the parameter changes will not be activated until the queries have completed. Also new queries will be held until the changes have been activated. Therefore, although the changes can be made when the system is live while the changes are being implemented the access to PDQ sub-system might be restricted.