3172: Remove implicit order by from query.
3172: Remove implicit order by from query.

This query was grouping by and not using any kind of order by. By default,
MySQL adds an order by with the same columns of the group by (this makes
the query need a filesort). From MySQL docs:

By default, MySQL sorts all GROUP BY col1, col2, ... queries as if you
specified ORDER BY col1, col2, ... in the query as well. If you include
an explicit ORDER BY clause that contains the same column list, MySQL
optimizes it away without any speed penalty, although the sorting still occurs.
If a query includes GROUP BY but you want to avoid the overhead of sorting
the result, you can suppress sorting by specifying ORDER BY NULL.

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