PDQPRIORITY

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PDQPRIORITY

The PDQPRIORITY environment variable determines the degree of parallelism that the database server uses and affects how the database server allocates resources, including memory, processors, and disk reads.

Extended Parallel Server

For Extended Parallel Server, the PDQPRIORITY environment variable determines only the allocation of memory resources.

End of Extended Parallel Server

>>-setenv--PDQPRIORITY--+-HIGH-----------------------------------------------+-><
                        +-LOW------------------------------------------------+
                        +-OFF------------------------------------------------+
                        '-resources--+-------------------------------------+-'
                                     '-Extended Server Only--,--high_value-'

resources: Is an integer in the range 0 to 100. Value 1 is the same as LOW.
Value 0 is the same as OFF (for supported database servers other than Extended Parallel Server only).
high_value: Optional integer value that requests the maximum percentage of memory (for Extended Parallel Server only). When you specify this value after the resources value, you request a range of memory, expressed as a percentage.

Here the HIGH, LOW, and OFF keywords have the following effects:

HIGH: When the database server allocates resources among all users, it gives as many resources as possible to the query.
LOW: Data is fetched from fragmented tables in parallel.
OFF: PDQ processing is turned off (for supported database servers other than Extended Parallel Server).

For supported database servers other than Extended Parallel Server, the resources value specifies the query priority level and the amount of resources that the database server uses to process the query. When you specify LOW, the database server uses no forms of parallelism.

Dynamic Server

When PDQPRIORITY is not set, the default value is OFF.

When PDQPRIORITY is set to HIGH, the database server determines an appropriate value to use for PDQPRIORITY based on several criteria. These include the number of available processors, the fragmentation of tables queried, the complexity of the query, and additional factors.

End of Dynamic Server

Extended Parallel Server

The resources value establishes the minimum percentage of memory when you also specify high_value to request a range of memory allocation. Other parallel operations can occur when the PDQPRIORITY setting is LOW.

When the PDQPRIORITY environment variable is not set, the default value is the value of the PDQPRIORITY configuration parameter.

When PDQPRIORITY is set to 0, the database server can execute a query in parallel, depending on the number of available processors, the fragmentation of tables queried, the complexity of the query, and so on. PDQPRIORITY does not affect the degree of parallelism in Extended Parallel Server.

End of Extended Parallel Server

Usually, the more resources a database server uses, the better its performance for a given query. If the server uses too many resources, however, contention among the resources can result and take resources away from other queries, resulting in degraded performance. For more information on performance considerations for PDQPRIORITY, refer to your IBM Informix: Performance Guide.

An application can override the setting of this environment variable when it issues the SQL statement SET PDQPRIORITY, as the IBM Informix: Guide to SQL Syntax describes.