--oracle索引失效 --创建测试表 create table t( empno number, ename varchar2(20), deptno number ); --创建索引 create index idx_deptno on t(deptno); --创建复合索引 create index idx_empno_deptno on t(empno,deptno); drop index idx_empno_deptno; create index idx_empno_deptno on t(deptno,empno); --插入测试数据 insert into t values(1001,'A',10); 1 <> SQL> select * from t where deptno=10; Execution Plan ---------------------------------------------------------- Plan hash value: 3776569808 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 2 | 76 | 1 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 2 | 76 | 1 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_DEPTNO | 2 | | 1 (0)| 00:00:01 | ------------------------------------------------------------------------------------------ SQL> select * from t where deptno<>10; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 7 | 266 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 7 | 266 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- 2 模糊查询 like '%_' SQL> select * from t where deptno like '%_1'; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 1 | 38 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- 3 单独引用复合索引里非第一位置的索引列. SQL> select * from t where empno=1005; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 1 | 38 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- 4 字符型字段为数字时在where条件里不添加引号 alter table t add id varchar2(20); create index idx_id on t(id); SQL> select * from t where id=33; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 50 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 1 | 50 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- SQL> select * from t where id='33'; Execution Plan ---------------------------------------------------------- Plan hash value: 827754323 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 50 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 50 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_ID | 1 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------- 5 对索引列进行运算.需要建立函数索引 SQL> select * from t where deptno*2=20; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 | 100 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 2 | 100 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- SQL> select * from t where substr(deptno,1,1)=3; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 50 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 1 | 50 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- 6 not in ,not exist. SQL> select * from t where deptno not in ('10','20','30','40','50','60'); Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 150 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 3 | 150 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- 7 基于cost成本分析(oracle因为走全表成本会更小)：查询小表,或者返回值大概在10%以上 8 B-tree索引 is null不会走,is not null会走 SQL> select * from t where id is null; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 7 | 350 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 7 | 350 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------- SQL> select * from t where id is not null; Execution Plan ---------------------------------------------------------- Plan hash value: 875909553 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 150 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 3 | 150 | 2 (0)| 00:00:01 | |* 2 | INDEX FULL SCAN | IDX_ID | 10 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------- 索引失效总结： 1.没有查询条件，查询条件没有建立索引 2.查询条件没有使用索引引导列 3.基于cost优化器，查询结果集30%以上 4.索引本身失效 5.查询条件使用函数在索引列上 6.隐式转换 7.模糊查询 8.not in , null ,<>