Union and Union All - processes

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Union and Union All - processes

Greetings Experts,

Can you please explain the following

The “union” command can be used to break up a large sql process or statement into several smaller sql processes or statements, which would run in parallel. But these could then cause spoolspace limit problems. “Union all” executes the sql’s single threaded

Why does union run using different processes unlike of union all which uses single process I guess.