``++UNION++`` is a convenient syntax to combine the results of two or more SQL statements because it helps you cut a complex problem into multiple simple SQL statements. But when it comes to execution, using ``++UNION++`` is debatable.
First, it may be possible to fuse two simple SQL statements into a bigger one that will run faster. Second, ``++UNION++`` is significantly less performant compared to ``++UNION ALL++`` because it removes duplicated entries and runS an internal ``++DISTINCT++`` to achieve this.
``++UNION ALL++`` does not remove duplicates and returns all the results from the queries. It performs faster in most cases compared to ``++UNION++``. Nevertheless, the quantity of data returned by ``++UNION ALL++`` can be significantly larger than with ``++UNION++``. On a slow network, the performance gain of using ``++UNION ALL++`` instead of ``++UNION++`` can be negated by the time lost in the larger data transfer.
This rule raises an issue on each ``++UNION++``. It's up to the developer to challenge its use and see if there is a better way to rewrite without ``++UNION++``.
