DBA_TABLESPACE_USAGE_METRICS Does Not Include Undo Tablespace Information (Doc ID 2491036.1)

Bug 28821847 : DBA_TABLESPACE_USAGE_METRICS IS NOT INCLUDING UNDO TABLESPACE INFO

zouxiang
感谢。
再请教下，我开始通过dba_segments查询undo表空间的sum（bytes/1024/1024)和dba_undo_extents的sum(bytes/1024/1024)大小是一致的；后来再查询这两个值发现通过dba_segments查询的值要比dba_undo_extents大了。这是什么原因？

How to monitor Undo Tablespace Usage and the Free Space (Doc ID 1951400.1)

The Undo Space once allocated wont be deallocated to the OS, by default. But the space can be reused by other transactions once the UNDO_RETENION (or TUNED_UNDORETENTION period) is met.

dba_tablespace_usage_metrics，使用比率，并不是对于表空间真实大小，而是相对于small tablespace单个datafile最大32GB即4194302的比率，单位为blocks：
也就是dba_tablespace_usage_metrics的used_percent=(used_space/TABLESPACE_SIZE)*100，所以查询dba_undo_extents更为准确一点。