TOP undo usage

Top 10 UNDO usage SQL
select SQL_TEXT from DBA_HIST_SQLTEXT where SQL_ID in
(select distinct MAXQUERYSQLID from
(select * from
(select SNAP_ID,
MAXQUERYLEN,
UNDOBLKS,
MAXQUERYSQLID,
to_char(BEGIN_TIME,’yyyy/mm/dd hh24:mi’) begin,
to_char(END_TIME,’yyyy/mm/dd hh24:mi’) end
from DBA_HIST_UNDOSTAT
order by UNDOBLKS desc,
MAXQUERYLEN desc
) where rownum<11
)
);