Benefits of Index-Organized Tables【每日一译】--20130101

Index-organized tables provide faster access to table rows by the primary key or any
key that is a valid prefix of the primary key. Presence of nonkey columns of a row in
the B-tree leaf block itself avoids an additional block access. Also, because rows are
stored in primary key order, range access by the primary key (or a valid prefix)

involves minimum block accesses.

索引组织表提供了更快的访问表行的方式通过主键或者其它可用的带前缀的主键。

一行的非主键列在B-TREE的叶子块中它自己避免了一个额外的块访问。同时行的存

的存储以主键的顺序，范围式访问通过主键（或者一个可用的前缀主键）涉及到了

最小块的访问。

In order to allow even faster access to frequently accessed columns, you can use a row
overflow segment (as described later) to push out infrequently accessed nonkey

columns from the B-tree leaf block to an optional (heap-organized) overflow segment.
This allows limiting the size and content of the portion of a row that is actually stored
in the B-tree leaf block, which may lead to a higher number of rows in each leaf block
and a smaller B-tree.

为了允许甚至更快的访问对于频繁访问的列，你可以使用行溢出段（稍后描述）去清除

非频繁访问的非主键列从B-TREE叶块到一个可选的（HASH-组织）溢出段。

这人允许限制大小和行中的部份内容，这些数据是实际存储在B-TREE叶子块的，它将导致

一个更高数量的行数在每个叶块和一个更小的B-TREE。

Unlike a configuration of heap-organized table with a primary key index where
primary key columns are stored both in the table and in the index, there is no such
duplication here because primary key column values are stored only in the B-tree
index.

不像一个HASH-组织的配置表有一个主键索引，这个主键列既存在于表也存在于索引，

但在IOT里面没有重复的值因为主键列的值仅存在B-TREE索引中。

Because rows are stored in primary key order, a significant amount of additional
storage space savings can be obtained through the use of key compression.
Use of primary-key based logical rowids, as opposed to physical rowids, in secondary
indexes on index-organized tables allows high availability. This is because, due to the
logical nature of the rowids, secondary indexes do not become unusable even after a
table reorganization operation that causes movement of the base table rows. At the
same time, through the use of physical guess in the logical rowid, it is possible to get
secondary index based index-organized table access performance that is comparable to
performance for secondary index based access to an ordinary table.

因为行的存储是根据主键的顺序来着，一个大量的额外存储空间的节省是可以获取的通过

使用主键压缩。主键的使用是基于逻辑ROWID，对应于物理ROWID，在IOT的二次索引是

允许高可用的。这是因为由于逻辑ROWID的类型，二次索引不会变得不可用甚至在一个表

重新组织操作，比如产生了基表行的移动。同时，通过物理的ROWID猜测逻辑ROWID，这是

可以获得二次索引表访问的性能，也就是比较于性能对于基于二次索引访问到一个常规表。