Currently SQL_BIG_RESULT is checked only at compile time.
 However, additional optimizations may take place after
 this check that change the sort method from 'filesort'
 to sorting via index. As a result the actual plan
 executed is not the one specified by the SQL_BIG_RESULT
 hint. Similarly, there is no such test when executing
 EXPLAIN, resulting in incorrect output.
 The patch corrects the problem by testing for
 SQL_BIG_RESULT both during the explain and execution
 phases.