Mapping Table (233)

The mapping table is a heap-organized table that stores logical rowids of the
index-organized table. Specifically, each mapping table row stores one logical rowid
for the corresponding index-organized table row. Thus, the mapping table provides
one-to-one mapping between logical rowids of the index-organized table rows and
physical rowids of the mapping table rows.

A bitmap index on an index-organized table is similar to that on a heap-organized
table except that the rowids used in the bitmap index on an index-organized table are
those of the mapping table as opposed to the base table. There is one mapping table for
each index-organized table and it is used by all the bitmap indexes created on that
index-organized table.

In both heap-organized and index-organized base tables, a bitmap index is accessed
using a search key. If the key is found, the bitmap entry is converted to a physical
rowid. In the case of heap-organized tables, this physical rowid is then used to access
the base table. However, in the case of index-organized tables, the physical rowid is
then used to access the mapping table. The access to the mapping table yields a logical
rowid. This logical rowid is used to access the index-organized table.

Though a bitmap index on an index-organized table does not store logical rowids, it is
still logical in nature.

映射表
1. 是一个堆组织表，保存的是索引组织表的逻辑 rowid
。也就是映射表在每一行中存储索引组织表内相应行的逻辑 rowid
2. 每个索引组织表只需要一个映射表，建立在索引组织表上的多个位图索引可以共享同一个映射表

[@more@]

来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/10599713/viewspace-983121/，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-983121/