Undo & rollback

Rollback

select 'The average of waits/gets is '||  
   to_char(round((sum(waits) / sum(gets)) * 100,2), 'FM00.99999')||'%' situation  
   , decode(sign(round((sum(waits) / sum(gets)) * 100,2)-2), 1, 'Add more rollback segments', 'OK') solution
From    v$rollstat

alter rollback segment rbs01 shrink;