Overview of Materialized Views【每日一译】--20121203

Materialized views are schema objects that can be used to summarize, compute,
replicate, and distribute data. They are suitable in various computing environments

such as data warehousing, decision support, and distributed or mobile computing:

物化视图是用户对象，它可以用于去汇总，计算，复制和分布数据。它们适合在多种计算机

运行协作环境比如数据仓库，决策支持，和分布或者移动计算。

■ In data warehouses, materialized views are used to compute and store aggregated
data such as sums and averages. Materialized views in these environments are
typically referred to as summaries because they store summarized data. They can
also be used to compute joins with or without aggregations. If compatibility is set
to Oracle9i or higher, then materialized views can be used for queries that include

filter selections.

#在数据仓库中，物化视图用于去计算和存储聚合数据比如求和平均值。物化视图在这些

环境是典型涉及汇总因为它们存储汇总数据。它们也可以用于包含或者不包含聚合的运算。

如果版本兼容性是9I或者更高，那么物化视图可以用于查询包含过滤的选择条件。

The optimizer can use materialized views to improve query performance by
automatically recognizing when a materialized view can and should be used to
satisfy a request. The optimizer transparently rewrites the request to use the
materialized view. Queries are then directed to the materialized view and not to
the underlying detail tables or views.

优化可以使用物化视图去提高查询性能使用自动的组织，当一个物化视图可以或

者将将用于满足要求时。优化器可以透明的重写要求通过使用物化视图。然后查询

可以直接的导入到物化视图而不是基层的细节表或者视图。

■ In distributed environments, materialized views are used to replicate data at
distributed sites and synchronize updates done at several sites with conflict
resolution methods. The materialized views as replicas provide local access to data
that otherwise has to be accessed from remote sites.
■ In mobile computing environments, materialized views are used to download a
subset of data from central servers to mobile clients, with periodic refreshes from
the central servers and propagation of updates by clients back to the central
servers.

#在分布式环境里，物化视图用于复制数据在分布式站点并且同步更新在几个站点的解决

方法。物化视图像复制品它提供了本地的访问数据，否则你将从远程的站点访问。

#在移动的计算环境里，物化视图用于下载一个数据的子集从中心服务器到移动客户端，

通过周期从中心服务器更新和更新传播到中心服务器通过客户端 。

Materialized views are similar to indexes in several ways:
■ They consume storage space.
■ They must be refreshed when the data in their master tables changes.
■ They improve the performance of SQL execution when they are used for query
rewrites.
■ Their existence is transparent to SQL applications and users.
Unlike indexes, materialized views can be accessed directly using a SELECT statement.
Depending on the types of refresh that are required, they can also be accessed directly
in an INSERT, UPDATE, or DELETE statement.
A materialized view can be partitioned. You can define a materialized view on a
partitioned table and one or more indexes on the materialized view.

物化视图在下面几个方式上是与索引相似的：

#它们占用存储空间

#它们必须刷新当有数据在主表中改变时。

#它们提高了SQL执行的性能当它们用于查询重写时。

#它们的存在对于SQL应用和用户来说是透明的。

不像索引，物化视图可以被直接访问通过使用一个SELECT语句。取决于它们要求的刷新类型，

它们也可以直接访问在一个INSERT,UPDATE,或者DELETE语句。

物化视图可以被分区的，你可以定义一个物化视图成一个分区表和一个或者多个索引在物化视图上。