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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.
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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