小编给大家分享一下如何检查Undo表空间使用情况，相信大部分人都还不怎么了解，因此分享这篇文章给大家参考一下，希望大家阅读完这篇文章后大有收获，下面让我们一起去了解一下吧！

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SELECT TABLESPACE_NAME, ROUND ( (USED / TOTAL_SIZE) * 100, 2) USED_RATE
  FROM (SELECT A.TABLESPACE_NAME, TOTAL_SIZE, USED
          FROM (  SELECT TABLESPACE_NAME,
                         ROUND (SUM (BYTES) / 1024 / 1024 / 1024, 2) TOTAL_SIZE
                    FROM DBA_DATA_FILES
                   WHERE 1 = 1 AND TABLESPACE_NAME LIKE 'UNDOTBS%'
                GROUP BY TABLESPACE_NAME) A,
               (  SELECT TABLESPACE_NAME,
                         ROUND (SUM (BYTES) / 1024 / 1024 / 1024, 2) USED
                    FROM DBA_UNDO_EXTENTS
                   WHERE 1 = 1 AND STATUS = 'UNEXPIRED'
                GROUP BY TABLESPACE_NAME) B
         WHERE 1 = 1 AND A.TABLESPACE_NAME = B.TABLESPACE_NAME);

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The Expired blocks will be reused and hence this should be counted as 'available' space in the Undo segment.

You can check the status of the undo extents via the SQLs below.
select sum(bytes /(1024*1024)) from dba_undo_extents where status='EXPIRED';
select sum(bytes /(1024*1024)) from dba_undo_extents where status='ACTIVE';
select sum(bytes /(1024*1024)) from dba_undo_extents where status='UNEXPIRED';

For more detailed information, please refer to：
Master Note: High Undo Space Usage ( Doc ID 1578639.1 )

Above all, if the undo status is Expired, you can safely ignore the undo usage, as the space will be reused automatically.

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