(sql_select.cc:2492) ==6919== by 0x8370179: mysql_explain_union(THD*, st_select_lex_unit*, select_result*) (sql_select.cc:16900) ==6919== by 0x82D8B36: execute_sqlcom_select(THD*, TABLE_LIST*) (sql_parse.cc:4658) ==6919== by 0x82DAA80: mysql_execute_command(THD*) (sql...