During st_select_lex_unit::prepare() the member select_unit*
st_select_lex_unit::union_result is being assigned to an instance
of one of the following classes:
 - select_unit
 - select_unit_ext
 - select_unit_recursive
 - select_union_direct
Select_union_direct used to pass the result of the query directly to
the receiving select_result without filling a temporary table. This class
wraps a select_result object and is currently used to process UNION ALL
queries. Other select_unit_* classes involve some additional result processing.
Pushed down units are processed on the engine side so the results must be
also passed directly to a select_result object. So in the case when
the unit pushdown is employed st_select_lex_unit::union_result must be
assigned to an instance of select_union_direct.