Bitmap Join Indexes (226)

In addition to a bitmap index on a single table, you can create a bitmap join index,
which is a bitmap index for the join of two or more tables. A bitmap join index is a
space efficient way of reducing the volume of data that must be joined by performing
restrictions in advance. For each value in a column of a table, a bitmap join index
stores the rowids of corresponding rows in one or more other tables. In a data
warehousing environment, the join condition is an equi-inner join between the
primary key column or columns of the dimension tables and the foreign key column
or columns in the fact table.

Bitmap join indexes are much more efficient in storage than materialized join views, an
alternative for materializing joins in advance. This is because the materialized join
views do not compress the rowids of the fact tables.

位图连接索引
1. 除了建立在单个表之上的位图索引，用户还可以在两个或多个表创建位图连接索引。位图连接索引
可以预先将有连接关系的数据进行保存，且所需的存储空间较小。对于一个表的某列的每个值，
位图连接索引为其保存其他表中与此值有连接关系的数据行的 rowid。
在数据仓库环境中，连接关系通常是维表中的主键与事实表中的外键进行等值内连接
2. 物化连接视图也是一种预先将连接物化的方法，但与之相比位图连接索引所需的存储空间更少。
因为物化连接视图不会压缩事实表中的 rowid。

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