Operator-level parallelism (or inter-operator parallelism) is a form of intra-query parallelism obtained by executing concurrently several operators of the same query. By contrast, intra-operator parallelism is obtained by executing the same operator on multiple processors, with each instance working on a different subset of data.
Parallelism has been a key focus of database research since the 1970s. For example, as early as 1978 Teradata was building highly-parallel database systems and quietly pioneered many of the ideas on parallel query execution . However, the intra-query parallelism employed by these early systems was mostly intra-operator or independent parallelism (see Classes of Parallelism below). Gamma  was one of the first database systems that allowed operator-level parallelism through pipelining.
Parallel processing uses multiple processors cooperatively to improve the performance of...
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