The ALL clause can speed up UNION statements

If you omit the ALL clause in an UNION statement, SQL Server must delete duplicate values, which in turn means that it has to sort the two sub-resultsets that have to be combined. Needless to say, this is a time-consuming operation.

In most cases, you decide whether to use the ALL clause depending on your application requirements, and therefore you have no choice. However, if you are 100% sure that the two sub-resultsets have no duplicate rows, you can sensibly speed up execution by adding an explicit ALL clause, as in:

SELECT Name, State FROM USCustomersUNION ALLSELECT Name, Country FROM AbroadCustomers
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