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.

對映表
1. 是一個堆組織表，儲存的是索引組織表的邏輯 rowid
。也就是對映表在每一行中儲存索引組織表內相應行的邏輯 rowid
2. 每個索引組織表只需要一個對映表，建立在索引組織表上的多個點陣圖索引可以共享同一個對映表

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