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 statement
SQL> 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.