constructor just store pointer to st_select_lex_union (SELECT_LEX_UNION).

prepare() call st_select_lex_unit::prepare.

fix_length_and_dec() prepare cache and receive type and parameters (maximum of length) of returning items (called only by Item_singlerow_subselect).

exec() call st_select_lex_unit::exec(). st_select_lex_unit::exec() can drop 'assigned' flag of Item_subselect if st_select_lex_unit::item is not 0.