Restrictions on asynchrony optimization

When the optimizer applies asynchrony optimization to a given query, some restrictions apply to the number of ATQs that can be used in the query execution plan.

The number of SHIPs or RPDs that are eligible to be coupled with an ATQ in a plan and benefit from asynchrony might be greater than either the maximum that is set by the FEDERATED_ASYNC parameter or the per server limit of the DB2_MAX_ASYNC_REQUESTS_PER_QUERY server option. In this case, the optimizer chooses SHIPs or RPDs to couple with an ATQ in such a way that:
  • The total number of ATQs in the plan is less than or equal to the value set for the following parameters, in the order listed:
    1. FEDERATED ASYNCHRONY special register, if specified
    2. FEDERATED_ASYNCHRONY bind or precompile option, if specified
    3. FEDERATED_ASYNC parameter
  • The total number of ATQs for a given server is less than or equal to the DB2_MAX_ASYNC_REQUESTS_PER_QUERY server option for that server.
  • The eligible SHIPs or RPDs can benefit from being coupled with an ATQ and improve query performance.