Oracle 视图 DBA_UNDO_EXTENTS 官方解释，作用，如何使用详细说明
本站中文解释
Oracle视图DBA_UNDO_EXTENTS用来查看表空间中所有在任何对象上使用的undo扩展空间的信息。这个视图提供了表空间名称、回滚空间名称、回滚段分配情况等相关信息。
通过使用这个视图，DBA可以查看表空间中正在使用的每一个回滚段的细节，这有助于及时发现使用的回滚段是否已被使用完毕或是回滚段存在可能满足的搁置/未使用并及时分配空间，并有效管理回滚段空间。
使用Oracle视图DBA_UNDO_EXTENTS的基本语法：
SELECT tablespace_name,undo_extents.segment_name
FROM dba_undo_extents;
上述语句用于查看表空间中所有使用的undo段信息，包括段名称和表空间名称。
官方英文解释
DBA_UNDO_EXTENTS describes the extents comprising the segments in all undo tablespaces in the database.
Note:
The status of the undo space distribution reported by DBA_UNDO_EXTENTS is correct for the undo tablespace that is active on the instance on which DBA_UNDO_EXTENTS is queried. However, due to the use of in-memory information that is different on each instance, there can be a discrepancy in the status of the undo space distribution of undo tablespaces active on other instances when queried from one instance. This does not affect undo functionality and is only a reporting discrepancy for other instances’ undo tablespace space distribution status. As a best practice, query the space distribution for an undo tablespace from the instance on which it is active.
| Column | Datatype | NULL | Description |
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Owner of the undo tablespace |
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Name of the undo segment |
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Name of the undo tablespace |
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ID of the extent |
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Absolute file number of the data file containing the extent |
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Start block number of the extent |
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Size of the extent (in bytes) |
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Size of the extent (in blocks) |
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Relative number of the file containing the segment header |
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Commit time of the undo in the extent expressed as Julian time. This column is deprecated, but retained for backward compatibility reasons. |
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Commit time of the undo in the extent expressed as Wallclock time.This column is deprecated, but retained for backward compatibility reasons. |
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Transaction Status of the undo in the extent:
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