Execution of Union in Oracle - Parallel or Sequential?

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How does a Union in Oracle execute ?

If I have 4 selects in my union statement, will the selects execute in parallel or sequential manner?

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1 Answers

It depends on Oracle database version you use and OPTIMIZER_FEATURES_ENABLE initialization parameter.

Shortly: if set to 12.1 (or higher), concurrent execution is enabled by default. If it is set to a lower version, concurrent execution can be enabled by the P1_CONCURRENT_UNION hint.

Documentation: Concurrent execution of UNION ALL says:

Traditionally, set operators are processed in a sequential manner. Individual branches can be processed in serial or parallel, but only one branch at a time, one branch after another. (...) The default behavior of concurrent execution for UNION or UNION ALL statements is controlled by the setting of the OPTIMIZER_FEATURES_ENABLE initialization parameter.

Read the whole document for more info. Also, search for the same subject in other Oracle database version documentation (if it differs from 12c whose link I posted).

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