DBA_EXTENTS describes the extents comprising the segments in all tablespaces in the database.
Note that if a datafile (or entire tablespace) is offline in a locally managed tablespace, you will not see any extent information. If an object has extents in an online file of the tablespace, you will see extent information about the offline datafile. However, if the object is entirely in the offline file, a query of this view will not return any records.
Related View
USER_EXTENTS describes the extents comprising the segments owned by the current user‘s objects. This view does not display the OWNER, FILE_ID, BLOCK_ID, or RELATIVE_FNO columns.
也就是说DBA_EXTENDS描述的是数据库所有表空间中段的扩展信息。使用DBA_EXTENDS必须保证相应的数据文件处于online状态，否则无法返回任何记录。对于一用于，常使用的是USER_EXTENTS视图，比DBA_EXTENDS视图少几个字段。
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DBA_SEGMENTS describes the storage allocated for all segments in the database.
Related View
USER_SEGMENTS describes the storage allocated for the segments owned by the current user‘s objects. This view does not display the OWNER, HEADER_FILE, HEADER_BLOCK, or RELATIVE_FNO columns.
DBA_SEGMENTS 视图描述的是数据库中所有段的存储和分配信息。
段包含多个扩展区，扩展区是与数据相关的用户可见的最底层的视图，通过这两个视图，我们可以知道一条记录所在的表空间，数据文件号和分区。还可以知道各储存段的大小和扩展区信息。
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