How Indexes Are Stored (206)

When you create an index, Oracle automatically allocates an index segment to hold the
index’s data in a tablespace. You can control allocation of space for an index’s segment
and use of this reserved space in the following ways:
■ Set the storage parameters for the index segment to control the allocation of the
index segment’s extents.
■ Set the PCTFREE parameter for the index segment to control the free space in the
data blocks that constitute the index segment’s extents.
The tablespace of an index’s segment is either the owner’s default tablespace or a
tablespace specifically named in the CREATE INDEX statement. You do not have to
place an index in the same tablespace as its associated table. Furthermore, you can
improve performance of queries that use an index by storing an index and its table in
different tablespaces located on different disk drives, because Oracle can retrieve both
index and table data in parallel.

索引如何存储
1. 通过下列方式控制索引段的空间分配和使用
a. 设置索引段的存储参数来控制如何为此索引段分配数据扩展
b. 为索引段设置 PCTFREE 参数，来控制组成数据扩展的各个数据块的可用空间情况。
2. Oracle能并行的retrieve索引和表 ， 所以将索引和表放在不同的两个物理磁盘上可改进性能

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