Overview of Indexes (194)

Indexes are optional structures associated with tables and clusters. You can create
indexes on one or more columns of a table to speed SQL statement execution on that
table. Just as the index in this manual helps you locate information faster than if there
were no index, an Oracle index provides a faster access path to table data. Indexes are
the primary means of reducing disk I/O when properly used.

You can create many indexes for a table as long as the combination of columns differs
for each index. You can create more than one index using the same columns if you
specify distinctly different combinations of the columns.

Oracle provides several indexing schemes, which provide complementary
performance functionality:
■ B-tree indexes
■ B-tree cluster indexes
■ Hash cluster indexes
■ Reverse key indexes
■ Bitmap indexes
■ Bitmap join indexes
Oracle also provides support for function-based indexes and domain indexes specific
to an application or cartridge.
The absence or presence of an index does not require a change in the wording of any
SQL statement. An index is merely a fast access path to the data. It affects only the
speed of execution. Given a data value that has been indexed, the index points directly
to the location of the rows containing that value.
Indexes are logically and physically independent of the data in the associated table.
You can create or drop an index at any time without affecting the base tables or other
indexes. If you drop an index, all applications continue to work. However, access of
previously indexed data can be slower. Indexes, as independent structures, require
storage space.
Oracle automatically maintains and uses indexes after they are created. Oracle
automatically reflects changes to data, such as adding new rows, updating rows, or
deleting rows, in all relevant indexes with no additional action by users.
Retrieval performance of indexed data remains almost constant, even as new rows are
inserted. However, the presence of many indexes on a table decreases the performance
of updates, deletes, and inserts, because Oracle must also update the indexes
associated with the table.
The optimizer can use an existing index to build another index. This results in a much
faster index build.

索引
1. 索引是可選結構 , 與表或簇有關
2. 使用者可以為一個表建立多個索引,只要不同索引使用的列或列的組合不同即可
3. 索引在物理上和邏輯上都與他對應的表是獨立的
4. 如果在一個表上建立了過多的索引,將降低其插入,更新,及刪除的效能

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