Why NOT Use My Index

为什么定义了索引，执行时却没有用到？ 可能的几种情况：

1.统计信息错误或者不完全。尝试重建统计信息或者调整统计信息采样比率（默认为10%）后再试试。

2.10g以后的版本，optimizer_mode默认为ALL_ROWS，即采用CBO的方式。如果optimizer_mode不为ALL_ROWS，就要看看，你的SQL是不是通过RBO的方法执行的。

SQL> show parameter optimizer_mode

NAME                                        TYPE        VALUE

------------------------------------ ----------- ------------------------------

optimizer_mode                         string       ALL_ROWS

关于optimizer_mode参数的解释可以参考： The Oracle SQL Optimizers

3.复合索引（Concatenated Indexes）中的前导列没有作为查询条件

针对两个以上列建立的索引被称作复合索引。组合索引的顺序对索引的使用有决定性的影响，建立时需仔细考虑。

推荐阅读： Concatenated Indexes

4.条件列包含函数但没有创建函数索引（Function-Based Indexes）

一般来讲，Where子句中使用了函数，即使列上有索引，也不会使用到。8i后引入了Function-Based Indexes，可以解决函数列用不到索引的问题。

推荐阅读：

Oracle Function Based Indexes

Function-Based Indexes

5.小表没必要使用索引

表内的记录数很小，使用索引有可能还没有全表扫描快。即使你建了索引Optimizer也可能选择全表扫描。

推荐阅读：

Index the Correct Tables and Columns

When not to use indexes

Indexes On Small Tables Part I（需FQ）

6.选择性(Selectivity)

理想的选择性值当然是1，即列中所有的值都是不同的，这样的字段建立索引的效果最好。随着选择性的下降（即列中的重复值越来越多），索引使用的效果也在下降。当低到一定程度，使用索引的效果反倒不如全表扫描来的快速了。

B*TREE Indexes improve the performance of queries that select a small percentage of rows from a table. As a general guideline, we should create indexes on tables that are often queried for less than   15% of the table's rows.  This value may be higher in situations where all data can be retrieved from an index, or where the indexed columns can be used for joining to other tables.

The ratio of the number of distinct values in the indexed column / columns to the number of records in the table represents the selectivity of an index. The ideal selectivity is 1. Such a selectivity can be reached only by unique indexes on NOT NULL columns.

Example with good Selectivity

A table having 100'000 records and one of its indexed column has 88000 distinct values, then the selectivity of this index is 88'000 / 10'0000 = 0.88.

Oracle implicitly creates indexes on the columns of all unique and primary keys that you define with integrity constraints. These indexes are the most selective and the most effective in optimizing performance. The selectivity of an index is the percentage of rows in a table having the same value for the indexed column. An index's selectivity is good if few rows have the same value.

Example with bad Selectivity

lf an index on a table of 100'000 records had only 500 distinct values, then the index's selectivity is 500 / 100'000 = 0.005 and in this case a query which uses the limitation of such an index will retum 100'000 / 500 = 200 records for each distinct value. It is evident that a full table scan is more efficient as using such an index where much more I/O is needed to scan repeatedly the index and the table.

推荐阅读：

How to measure Index Selectivity

Index the Correct Tables and Columns

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