ap_query_replica_fallback

Description

ap_query_replica_fallback specifies whether to fall back to non-columnar replicas when no columnar replicas are available for an AP query that is automatically routed.

Privilege requirements

• Query variables

  • Global level

    sys tenants and all user tenants can execute the SHOW VARIABLES statement or query the SYS.TENANT_VIRTUAL_GLOBAL_VARIABLE view (Oracle mode) or the information_schema.GLOBAL_VARIABLES view (MySQL mode) to view the value of a global system variable.

  • Session level

    sys tenants and all user tenants can execute the SHOW VARIABLES statement or query the SYS.TENANT_VIRTUAL_SESSION_VARIABLE view (Oracle mode) or the information_schema.SESSION_VARIABLES view (MySQL mode) to view the value of a session system variable.

• Modify variables

  • Global level

    • sys tenants can directly modify the value of a global system variable.

    • MySQL user tenants must have the SUPER or ALTER SYSTEM privilege to modify the value of a global system variable.

    • Oracle user tenants must have the ALTER SYSTEM privilege to modify the value of a global system variable.

  • Session level

    sys tenants and all user tenants can directly modify the value of a session system variable for their own tenant.

Attributes

Usage

The ap_query_replica_fallback variable specifies whether to fall back to non-columnar replicas when no columnar replicas are available for an AP query that is automatically routed.

• When the value is ON: If the AP/TP automatic routing feature is enabled, the optimizer checks whether columnar replicas are available before generating a columnar replica plan for an AP query. If any of the tables involved in the query do not have available columnar replicas, the optimizer does not generate a columnar replica plan and instead falls back to non-columnar replicas.
• When the value is OFF: If the AP/TP automatic routing feature is enabled, the optimizer does not check whether columnar replicas are available and directly generates a columnar replica plan for an AP query. If columnar replicas are unavailable, an error is returned during query execution.

This variable works in conjunction with the ap_query_route_policy variable. If the value of ap_query_route_policy is OFF, this variable has no effect.

Examples

Set the value of ap_query_replica_fallback to ON.

• Session level

  obclient> SET ap_query_replica_fallback = ON;
  

• Global level

  obclient> SET GLOBAL ap_query_replica_fallback = ON;
  

References

ap_query_route_policy