索引与Null值对于Hints及执行计划的影响

由于B*Tree索引不存储Null值,所以在索引字段允许为空的情况下,某些Oracle查询不会使用索引.

很多时候,我们看似可以使用全索引扫描(Full Index Scan)的情况,可能Oracle就会因为Null值的存在而放弃索引.

在此情况下即使使用Hints,Oracle也不会使用索引,其根本原因就是因为Null值的存在.

我们看以下测试.

在username字段为Not Null时,Index Hints可以生效.

SQL> create table t as select username,password from dba_users; Table created. SQL> desc t Name Null? Type ----------------------------------------- -------- ---------------------------- USERNAME NOT NULL VARCHAR2(30) PASSWORD VARCHAR2(30) SQL> create index i_t on t(username); Index created. SQL> set autotrace trace explain SQL> select * from t where username='EYGLE'; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 34 | 2 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| T | 1 | 34 | 2 (0)| 00:00:01 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("USERNAME"='EYGLE') Note ----- - dynamic sampling used for this statement SQL> set linesize 120 SQL> select /*+ index(t,i_t) */ * from t where username='EYGLE'; Execution Plan ---------------------------------------------------------- Plan hash value: 2928007915 ------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 34 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 34 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | I_T | 1 | | 1 (0)| 00:00:01 | ------------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("USERNAME"='EYGLE') Note ----- - dynamic sampling used for this statement

当索引字段允许为Null时,Oracle放弃此索引:

SQL> alter table t modify (username null); Table altered. SQL> select /*+ index(t,i_t) */ * from t; Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 27 | 918 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS FULL| T | 27 | 918 | 2 (0)| 00:00:01 | -------------------------------------------------------------------------- Note ----- - dynamic sampling used for this statement

当该字段为Not Null时,索引可以被强制使用:

SQL> alter table t modify (username not null); Table altered. SQL> select /*+ index(t,i_t) */ * from t; Execution Plan ---------------------------------------------------------- Plan hash value: 3593393735 ------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 27 | 918 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 27 | 918 | 2 (0)| 00:00:01 | | 2 | INDEX FULL SCAN | I_T | 27 | | 1 (0)| 00:00:01 | ------------------------------------------------------------------------------------ Note ----- - dynamic sampling used for this statement

这就是Null值对于索引及查询的影响.