Bitmap Indexes (220)

The purpose of an index is to provide pointers to the rows in a table that contain a
given key value. In a regular index, this is achieved by storing a list of rowids for each
key corresponding to the rows with that key value. Oracle stores each key value
repeatedly with each stored rowid. In a bitmap index, a bitmap for each key value is
used instead of a list of rowids.

Each bit in the bitmap corresponds to a possible rowid. If the bit is set, then it means
that the row with the corresponding rowid contains the key value. A mapping
function converts the bit position to an actual rowid, so the bitmap index provides the
same functionality as a regular index even though it uses a different representation
internally. If the number of different key values is small, then bitmap indexes are very
space efficient.

Bitmap indexing efficiently merges indexes that correspond to several conditions in a
WHERE clause. Rows that satisfy some, but not all, conditions are filtered out before the
table itself is accessed. This improves response time, often dramatically.

點陣圖索引
1. 點陣圖索引只需儲存每個鍵值的點陣圖來代替一組ROWID
2. Oracle 透過一個對映函式將位資訊轉化為實際的 ROWID 。如果索引鍵不同的值數量較少時，
點陣圖索引的儲存效率相當高
3. 如果在 WHERE 子句內引用的多個列上都建有點陣圖索引 ，在訪問表之前不滿足全部條件的行可以被預先過濾掉，
從而極大地提高查詢的響應時間

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