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We decompose PhysicalPlan into Pipelines. A pipeline is a linear sequence of physical operators. The leaf operator of a pipeline is a source operator that scans from disk, storage or the output of other pipelines. The last operator of a pipeline is a sink operator that accumulates the intermediate results of the pipeline. Within a pipeline, data flows between operators without materialization until sink.

Pipeline decomposition

Given a physical plan, we decompose into pipelines when we encounter a sink operator. A sink operator is an operator that must exhaust its input in order to process correctly, e.g. HASH_JOIN_BUILD, AGGREGATE, ORDER BY, etc. Pipelines have dependencies, meaning that one pipeline may depend on the output of another pipeline. For e.g., HASH_JOIN_PROBE pipeline must depend on a HASH_JOIN_BUILD pipeline.

Morsel-driven parallelism

A pipeline can be executed with multiple threads. The granularity of multi-threading is controlled by its source operator that is responsible of dispatching source data to different threads’ pipelines. For e.g., to scan a CSV file in parallel, the source operator will first the count number of lines in the file and dispatch different lines to different threads.