free space in UNDO tablespace

select a.tablespace_name, SIZEMB, USAGEMB, (SIZEMB - USAGEMB) FREEMB
from (select sum(bytes) / 1024 / 1024 SIZEMB, b.tablespace_name
from dba_data_files a, dba_tablespaces b
where a.tablespace_name = b.tablespace_name
and b.contents = 'UNDO'
group by b.tablespace_name) a,
(select c.tablespace_name, sum(bytes) / 1024 / 1024 USAGEMB
from DBA_UNDO_EXTENTS c
where status <> 'EXPIRED'
group by c.tablespace_name) b
where a.tablespace_name = b.tablespace_name;

  • session using lot of undo
select a.sid, a.serial#, a.username, b.used_urec used_undo_record, b.used_ublk used_undo_blocks
from v$session a, v$transaction b
where a.saddr=b.ses_addr ;

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