Table Compression【每日一译】--20121124

Oracle's table compression feature compresses data by eliminating duplicate values in
a database block. Compressed data stored in a database block (also known as disk
page) is self-contained. That is, all the information needed to re-create the
uncompressed data in a block is available within that block. Duplicate values in all the
rows and columns in a block are stored once at the beginning of the block, in what is
called a symbol table for that block. All occurrences of such values are replaced with a

short reference to the symbol table.

ORACLE的表的压缩特点压缩数据通过使用清除重复数值在一个数据库块里。压缩数据存储

在一个数据块（另外一种是磁盘页）是自己包含的。也就是所有的信息需要去重建未压缩的数据

在一个数据块中它是可用的在那个块里面。在所有行和所有列中的重复值只存储一次在块的开头，

对于那个块来说这个叫作是符号表。所有这些值的变异的替换通过一个快捷的与符号表相关的操作。

With the exception of a symbol table at the beginning, compressed database blocks
look very much like regular database blocks. All database features and functions that

work on regular database blocks also work on compressed database blocks.

除了符号表开头的例外，压缩数据库块看起来很像普通的数据库块。所有数据库特性和功能上，压缩数据

库块和普通数据库块都是相同的。

Database objects that can be compressed include tables and materialized views. For

partitioned tables, you can choose to compress some or all partitions. Compression

attributes can be declared for a tablespace, a table, or a partition of a table. If declared
at the tablespace level, then all tables created in that tablespace are compressed by
default. You can alter the compression attribute for a table (or a partition or
tablespace), and the change only applies to new data going into that table. As a result,
a single table or partition may contain some compressed blocks and some regular
blocks. This guarantees that data size will not increase as a result of compression; in
cases where compression could increase the size of a block, it is not applied to that
block.

数据库对象可以被压缩对象包括表，物化视图。对于分区表，你可以选择部份或全部的分区表。

压缩属性可以定义在表空间的级别，那样话所有在这个表空间上新建的表默认是被压缩的。你可以

修改一个表的压缩属性（或者一个分区或者是表空间），并且修改应用只对于后面在这个表中的新

增数据。结果就是，一个单独表或者分区可以包含部分压缩块和一些普通的数据块。这确保了压缩

结果不增加数据量的大小。有些情况是压缩可能会增加块的大小，压缩不应用这种块；