Overview of Clusters (238)

Clusters are an optional method of storing table data. A cluster is a group of tables
that share the same data blocks because they share common columns and are often
used together. For example, the employees and departments table share the
department_id column. When you cluster the employees and departments
tables, Oracle physically stores all rows for each department from both the
employees and departments tables in the same data blocks.

Because clusters store related rows of different tables together in the same data blocks,
properly used clusters offers these benefits:
■ Disk I/O is reduced for joins of clustered tables.
■ Access time improves for joins of clustered tables.
■ In a cluster, a cluster key value is the value of the cluster key columns for a
particular row. Each cluster key value is stored only once each in the cluster and
the cluster index, no matter how many rows of different tables contain the value.
Therefore, less storage is required to store related table and index data in a cluster
than is necessary in nonclustered table format. For example, in Figure 5–10, notice
how each cluster key (each department_id) is stored just once for many rows
that contain the same value in both the employees and departments tables.

簇
1. 簇由一组拥有相同的列且经常被一起使用的数据表构成，这组表在存储时可共享数据块
2. 簇有以下的优点
a. 连接簇表所需的磁盘 I/O 会减少
b. 连接簇表所需的时间将减少
c. 在一个簇中，簇键值是指簇键列distinct的值。一个簇内的由多个簇表的各个数据行所使用的相同的簇键值，
在簇及簇索引中只会被存储一次。因此所需的存储空间更少。

[@more@]

来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/10599713/viewspace-983126/，如需转载，请注明出处，否则将追究法律责任。

转载于:http://blog.itpub.net/10599713/viewspace-983126/