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The document describes internal information about the Cisco Information Server (CIS) query engine, in case you must troubleshoot performance from a query that involves UNIONs.
When you run a query that contains a UNION (without the ALL), it essentially first performs a UNION ALL.
Afterwards, the CIS query engine runs a hidden DISTINCT operator that removes the duplicate records. This duplicate removal is done in the memory or spills to disk if memory is exhausted.