ORDER BY computed col
      
GROUP BY implies ORDER BY in the MySQL dialect of SQL. Therefore, when an
index on the first table in the query is used, and that index satisfies
ordering according to the GROUP BY clause, the query optimizer estimates the
number of tuples that need to be read from this index. If there is a LIMIT
clause, table statistics on tables following this 'sort table' are employed.

There may be a separate ORDER BY clause however, which mandates reading the
whole 'sort table' anyway. But the previous estimate was left untouched.

Fixed by removing the estimate from EXPLAIN output if GROUP BY is used in
conjunction with an ORDER BY clause that mandates using a temporary table.