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Composite Indexes (196)

原创 数据挖掘 作者:tsinglee 时间:2007-11-14 10:03:07 0 删除 编辑

A composite index (also called a concatenated index) is an index that you create on
multiple columns in a table. Columns in a composite index can appear in any order
and need not be adjacent in the table.

Composite indexes can speed retrieval of data for SELECT statements in which the
WHERE clause references all or the leading portion of the columns in the composite
index. Therefore, the order of the columns used in the definition is important.
Generally, the most commonly accessed or most selective columns go first.

No more than 32 columns can form a regular composite index. For a bitmap index, the
maximum number columns is 30. A key value cannot exceed roughly half (minus
some overhead) the available data space in a data block.

复合索引(连接索引)
1. 合索引内的列可以任意排列,她们在数据表中也无需相邻.
2. 如果一个 SELECT 语句的 WHERE 子句中引用了复合索引的全部列或自首列开始且连续的部分列,将有助于提高此查询的性能
3. 一个常规的复合索引不能超过 32 列,而位图索引不能超过 30 列.

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