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.

點陣圖連線索引
1. 除了建立在單個表之上的點陣圖索引，使用者還可以在兩個或多個表建立點陣圖連線索引。點陣圖連線索引
可以預先將有連線關係的資料進行儲存，且所需的儲存空間較小。對於一個表的某列的每個值，
點陣圖連線索引為其儲存其他表中與此值有連線關係的資料行的 rowid。
在資料倉儲環境中，連線關係通常是維表中的主鍵與事實表中的外來鍵進行等值內連線
2. 物化連線檢視也是一種預先將連線物化的方法，但與之相比點陣圖連線索引所需的儲存空間更少。
因為物化連線檢視不會壓縮事實表中的 rowid。

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