parallel_servers_target

Description

parallel_servers_target specifies the parallel query queuing conditions for each server.

When the total number of threads consumed by multiple parallel executions (Parallel eXecution, PX) exceeds the parallel query queuing conditions, subsequent parallel execution SQL statements must wait in a queue.

Privilege requirements

• Query variables

  The sys tenant and all user tenants can use the SHOW VARIABLES statement or the SYS.TENANT_VIRTUAL_GLOBAL_VARIABLE view (Oracle mode) and the information_schema.GLOBAL_VARIABLES view (MySQL mode) to view the values of global system variables.

• Modify variables

  • The sys tenant can directly modify the values of global system variables.

  • MySQL user tenants need to have the SUPER or ALTER SYSTEM privilege to modify the values of global system variables.

  • Oracle user tenants need to have the ALTER SYSTEM privilege to modify the values of global system variables.

Attributes

Usage notes

parallel_servers_target is a tenant-level variable. When a tenant is created:

1. If the parallel_servers_target value is specified in the CREATE TENANT statement, the tenant uses the specified value to set parallel_servers_target.

   The statement for specifying the parallel_servers_target value when creating a tenant is as follows:

   obclient> CREATE TENANT IF NOT EXISTS <tenant_name> CHARSET='utf8mb4', ZONE_LIST=('zone1'), PRIMARY_ZONE='zone1', RESOURCE_POOL_LIST=('<pool_name>') set parallel_servers_target = 624;
   

2. If the parallel_servers_target value is not specified when creating a tenant, the value is automatically calculated based on the min_cpu parameter during the tenant creation process on the OBServer node.

   You can log in to the tenant and use the following statement to view the value:

   obclient> show variables like 'parallel_servers_target';
   

   The automatically calculated parallel_servers_target value is usually small and can only meet the needs of simple parallel queries. If you want to use parallel operations in this tenant, we recommend that you manually set the value to a larger value. The recommended algorithm is parallel_servers_target = max_cpu * 10.

Examples

Set the parallel query queuing condition to 624 for each server.

obclient> SET GLOBAL parallel_servers_target = 624;

References

Optimize the parameters for parallel query