统一段，扩展，块segments, extents, and data blocks.

Starting with the highest level of Oracle disk-space management are tablespaces. Drilling down,
you find segments that can reside in only one tablespace. Each segment is constructed from one
or more extents . Each of these extents can reside in only one datafile. Thus, for a segment to
straddle multiple datafiles, it must be constructed from multiple extents, which are located in
separate datafiles. An extent is composed of a contiguous set of data blocks, which is at the lowest
level of space management. A data block is a fixed number of bytes of disk space.

Figure 3.1 shows the relationship among segments, extents, and data blocks.

Oracle磁盘空间管理的最高水平开始的表空间。向下探索，你会发现一个表空间可以驻留多个段。每个段可以由一个或多个扩展构造。这些扩展，每个都可以参存放一个数据文件。

因此，对于一个段来说，跨越多个数据文件，它必须onstructed从多个扩展，这是在位于不同的数据文件。在一定程度上是由一个连续的数据块，这是在空间管理的最低水平。一个数据块是一个固定数目的字节的磁盘空间。图3.1显示了段，程度和数据块之间的关系。