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A normal SQL processing without the parallel query feature is always performed by a single server process.
With the feature enabled, multiple processes are created to simultaneously work together on that single SQL statement and can dramatically improve the performance of data-intensive operations such as those in very large database systems.
Since the many processes can be split among multiple CPUs present, clustered and massively parallel systems, as well as symmetric multiprocessing systems can greatly benefit from it.
Parallel query processing occurs during the query optimization phase where the SQL server is looking for query index operations which might benefit from parallel operation.
When found, the server inserts exchange operators into the query execution plan in order to turn it into a parallel-query execution plan which can use more than one thread or process for execution.