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 explainSQL> 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 statementSQL> set linesize 120SQL> 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.