segment（段） 包含 extent 盘区 包含block 数据块
In summary, the hierarchy of storage in Oracle is as follows:
1. A database is made up of one or more tablespaces.
2. A tablespace is made up of one or more data files. A tablespace contains segments.
3. A segment (TABLE, INDEX, and so on) is made up of one or more extents. A
segment exists in a tablespace, but may have data in many data files within that
4. An extent is a contiguous set of blocks on disk. An extent is in a single tablespace
and furthermore, is always in a single file within that tablespace.
5. A block is the smallest unit of allocation in the database. A block is the smallest unit
of I/O used by a database.
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