--oracle索引失效 --創建測試表 create table t( empno number, ename varchar2(20), deptno number ); --創建索引 create index idx_deptno on t(deptno); --創建復合索引 create index idx_empno_deptno on t(empno,deptno); drop index idx_empno_deptno; create index idx_empno_deptno on t(deptno,empno); --插入測試數據 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 模糊查詢 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 單獨引用復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 字符型字段為數字時在where條件里不添加引號 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 對索引列進行運算.需要建立函數索引 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 基於cost成本分析(oracle因為走全表成本會更小)：查詢小表,或者返回值大概在10%以上 8 B-tree索引 is null不會走,is not null會走 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 | -------------------------------------------------------------------------------------- 索引失效總結： 1.沒有查詢條件，查詢條件沒有建立索引 2.查詢條件沒有使用索引引導列 3.基於cost優化器，查詢結果集30%以上 4.索引本身失效 5.查詢條件使用函數在索引列上 6.隱式轉換 7.模糊查詢 8.not in , null ,<>