Sets upper bound for parallel execution of GROUP BY operations and for arms of a UNION.
Option can be set at the database (PUBLIC) or user level. When set at the database level, the value becomes the default for any new user, but has no impact on existing users. When set at the user level, overrides the PUBLIC value for that user only. No system privilege is required to set option for self. System privilege is required to set at database level or at user level for any user other than self.
Requires the SET ANY PUBLIC OPTION system privilege to set this option. Can be set temporary for an individual connection or for the PUBLIC role. Takes effect immediately.
This parameter sets an upper bound which limits how parallel the optimizer will permit query operators to go. This can influence the CPU usage for many query join, GROUP BY, UNION, ORDER BY, and other query operators.
Systems with more than 64 CPU cores often benefit from a larger value, up to the total number of CPU cores on the system to a maximum of 512; you can experiment to find the best value for this parameter for your system and queries.
Systems with 64 or fewer CPU cores should not need to reduce this value, unless excessive system time is seen. In that case, you can try reducing this value to determine if that adjustment can lower the CPU system time and improve query response times and overall system throughput.