``++UNION++`` is significantly less performant compared to ``++UNION ALL++`` because it removes duplicated entries and run an internal ``++DISTINCT++`` to achieve this.
``++UNION ALL++`` is not removing duplicates and returns all the results from the queries. It performs faster in most of the cases compared to ``++UNION++``. Nevertheless, the quantity of data returned by ``++UNION ALL++`` can be significantly more important than with ``++UNION++``. On slow network, the performance gain to use ``++UNION ALL++`` instead of ``++UNION++`` can be challenged by the time lost to transfer more data than with ``++UNION++``.
