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.

索引如何儲存
1. 透過下列方式控制索引段的空間分配和使用
a. 設定索引段的儲存引數來控制如何為此索引段分配資料擴充套件
b. 為索引段設定 PCTFREE 引數，來控制組成資料擴充套件的各個資料塊的可用空間情況。
2. Oracle能並行的retrieve索引和表 ， 所以將索引和表放在不同的兩個物理磁碟上可改進效能

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