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Instance 2 ->  INSTANCE_GROUPS = g_12, g_23, g_24, g_123
PARALLEL_INSTANCE_GROUP = g_123


Instance 3 ->  INSTANCE_GROUPS = g_13, g_23, g_34
PARALLEL_INSTANCE_GROUP = g_34

Instance 4 ->  INSTANCE_GROUPS = g_14, g_24, g_34
PARALLEL_INSTANCE_GROUP = g_12

When a user logs on instance 1 and executes a parallel SQL operation (without issuing an alter session set parallel_instance_group command), the operation will be executed by parallel server processes from instances 1 and 2. However, if the user executes

alter session set parallel_instance_group g_123 ;

before issuing the SQL statement, the operation will be executed on instances 1, 2, and 3.

Tuning Parallel Query

Parallel Query Option is a very powerful tool and, if used effectively, it can reduce the processing time by several orders. However, it is very resource consuming, and the achieved performance gain is highly dependent on the data distribution. You should have enough CPU and memory resources to support the active parallel query slaves. Also, plan your data appropriately so that you do not have an I/O bottleneck during parallel processing.l

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