ap_query_cost_threshold

Description

ap_query_cost_threshold specifies the cost threshold for adaptive selection of columnar replicas.

Privilege requirements

• Query variables

  • Global level

    The sys tenant and all user tenants can query the value of Global system variables by using the SHOW VARIABLES statement or the SYS.TENANT_VIRTUAL_GLOBAL_VARIABLE view (Oracle-compatible mode) and the information_schema.GLOBAL_VARIABLES view (MySQL-compatible mode).

  • Session level

    The sys tenant and all user tenants can query the value of Session system variables by using the SHOW VARIABLES statement or the SYS.TENANT_VIRTUAL_SESSION_VARIABLE view (Oracle-compatible mode) and the information_schema.SESSION_VARIABLES view (MySQL-compatible mode).

• Modify variables

  • Global level

    • The sys tenant can directly modify the value of Global system variables.

    • MySQL user tenants must have the SUPER or ALTER SYSTEM privilege to modify the value of Global system variables.

    • Oracle user tenants must have the ALTER SYSTEM privilege to modify the value of Global system variables.

  • Session level

    The sys tenant and all user tenants can directly modify the value of Session system variables.

Attributes

Usage

A parallel SELECT plan is generated based on the rules. When the cost of a rowstore replica SELECT plan with a single parallelism level exceeds the value of the ap_query_cost_threshold variable, the system generates a plan for a columnar replica.

Examples

Set the value of ap_query_cost_threshold to 100000.

• Session level

  obclient> SET ap_query_cost_threshold = 100000;
  

• Global level

  obclient> SET GLOBAL ap_query_cost_threshold = 100000;
  

References

ap_query_route_policy