PCTFREE, PCTUSED, and Row Chaining【每日一译】--2012-10-20

For manually managed tablespaces, two space management parameters, PCTFREE

and PCTUSED, enable you to control the use of free space for inserts and updates to the

rows in all the data blocks of a particular segment. Specify these parameters when you

create or alter a table or cluster (which has its own data segment). You can also specify

the storage parameter PCTFREE when creating or altering an index (which has its own

index segment).

对于人工管理的表空间，两种 空间管理参数，PCTFREE和PCTUSED,让你去控制空闲空间的

使用，对于插入和更新一个指定段的数据块的行。指定这些参数当你创建或更改一个表或集群

（它拥有它自己的数据段）。你也可以指定存储参数PCTFREE当创建或者改变一个索引（它

拥有它自己的索引段）

Note: This discussion does not apply to LOB datatypes (BLOB,

CLOB, NCLOB, and BFILE). They do not use the PCTFREE storage

parameter or free lists.

See "Overview of LOB Datatypes" on page 26-10 for information.

备注：这个讨论不能作用于LOB数据类型（BLOB,CLOB,NCLOB,和BFILE），它们不使用PCTFREE

参数或者是自由列表

The PCTFREE Parameter

The PCTFREE parameter sets the minimum percentage of a data block to be reserved

as free space for possible updates to rows that already exist in that block. For example,

assume that you specify the following parameter within a CREATE TABLE statement:

PCTFREE参数设置最小的数据块百分比预留已经存在的数据块可能做的更新需要的空闲

空间。比如，假定你指定了以下参数在CREATE TABLE语句：

PCTFREE 20

This states that 20% of each data block in this table’s data segment be kept free and

available for possible updates to the existing rows already within each block. New

rows can be added to the row data area, and corresponding information can be added

to the variable portions of the overhead area, until the row data and overhead total

80% of the total block size. Figure 2–3illustrates PCTFREE.

PCTFREE 20

这个20%的状态是对于表的数据段的每个块保留空闲对于在每个块的存在行的可能更新。

新行的数据可以被添加到行数据区或，并且对应的信息可以被添加到消耗区域的可用的部份

直到行数据和消耗掉数据块总共大小的80%。图表2-3显示了PCTFREE



The PCTUSED Parameter

The PCTUSED parameter sets the minimum percentage of a block that can be used for

row data plus overhead before new rows are added to the block. After a data block is

filled to the limit determined by PCTFREE, Oracle considers the block unavailable for

the insertion of new rows until the percentage of that block falls beneath the parameter

PCTUSED. Until this value is achieved, Oracle uses the free space of the data block only

for updates to rows already contained in the data block. For example, assume that you

specify the following parameter in a CREATE TABLE statement:

PCTUSED参数设置最小的数据块百分比用于行数加上消耗。直到达到这个值，ORACLE

使用数据块的空闲空间仅用于更新在已经包含于的数据块。比如，假定设定了以下参数在

CREATE TABLE 语句：

PCTUSED 40

In this case, a data block used for this table’s data segment is considered unavailable

for the insertion of any new rows until the amount of used space in the block falls to

39% or less (assuming that the block’s used space has previously reached PCTFREE).

Figure 2–4 illustrates this.

PCTUSED 40

在这种情况，一个数据块对于表的数据段是被认为不可用的对于插入任何的新行直到这个

块的使用空间数量低于39%或者更少（假定这个数据块的使用空间先前已经达到PCTFREE）

图表2-4显示了这些。