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The Concatenation Operator

Hugo Kornelis explains what the Concatenation operator does:

The Concatenation operator reads and returns all rows from all its inputs, in order, and without modification.

This operator is most commonly used to execute queries that use UNION or UNION ALL. In the former case, other operators are required to remove the duplicates, because Concatenation doesn’t provide that functionality. You may also find the Concatenation operator in queries on partitioned views.

Read on to see the algorithm and lots of details about the operator.