Figure 21-1 illustrates message buffers and how producer parallel execution servers connect to consumer parallel execution servers.
Text description of the illustration dwhsg096When a connection is between two processes on the same instance, the servers communicate by passing the buffers back and forth.
A single-instance environment uses at most three buffers for each communication channel.
When a user issues a SQL statement, the optimizer decides whether to execute the operations in parallel and determines the degree of parallelism (DOP) for each operation.
You can specify the number of parallel execution servers required for an operation in various ways.
Each server in the producer execution process set has a connection to each server in the consumer set.
This means that the number of virtual connections between parallel execution servers increases as the square of the DOP.
Parallel execution improves processing for: You can also use parallel execution to access object types within an Oracle database.
For example, you can use parallel execution to access large objects (LOBs).
During the day, most OLTP systems should probably not use parallel execution.
During off-hours, however, parallel execution can effectively process high-volume batch operations.
When executing a parallel operation, the parallel execution coordinator obtains parallel execution servers from the pool and assigns them to the operation.