Oracle Parallel Processing

Instance Groups for Parallel Execution

While parallelizing SQL statements in an OPS environment, Oracle doesn’t have to use all of the OPS instances. You can use instance groups to specify which instances to use.

We introduced instance groups in Chapter 7. One of the major applications of instance groups is to specify the instances to use for parallel execution. Assuming that you have used the INSTANCE_GROUPS initialization parameter to group your OPS instances, you can then use the PARALLEL_INSTANCE_GROUP initialization parameter to identify the group that you want Oracle to use when it executes a SQL statement in parallel. This parameter can be specified in the initialization parameter file as shown in this example:

PARALLEL_INSTANCE_GROUP = groupB

All parallel operations initiated from this instance will spawn parallel slave processes on the instances belonging to the group named groupB.

The PARALLEL_INSTANCE_GROUP parameter can be changed while an instance is running using the ALTER SESSION or ALTER SYSTEM command, for example:

ALTER SESSION SET PARALLEL_INSTANCE_GROUP = 'groupB';
ALTER SYSTEM SET PARALLEL_INSTANCE_GROUP = 'groupB';

ALTER SESSION changes the parameter for the current session only, whereas ALTER SYSTEM changes the setting for the current instance. Any changes to the setting using ALTER SYSTEM take effect immediately for all existing sessions of the instance.

The default setting for the PARALLEL_INSTANCE_GROUP parameter is a group composed of all the running instances ...