NULL值是关系数据库系统布尔型(true,false,unknown)中比较特殊类型的一种值,通常称为UNKNOWN或空值,即是未知的,不确定的。由于 NULL存在着无数的可能,因此NULL值也不等于NULL值,所以与NULL值相关的操作同样都为NULL值。正是基于这样一个特性,对于NULL值列上的B 树索引导致了is null/is not null不走索引的情形,下面描述了NULL值与索引以及索引NULL列上的执行计划,如何使得NULL值走索引的情形。 注:本文仅仅讨论的是B树索引上的NULL值,位图索引不在此范围之内。 一、null值与索引的关系
scott@ORCL> create table t1(id number,val varchar2(1));
-->为表t1创建唯一索引
scott@ORCL> create unique index i_t1_id on t1(id);
scott@ORCL> insert into t1 select null,'Y' from dual;
scott@ORCL> insert into t1 select null,'N' from dual;
-->从上面的操作可知,尽管列id上存在唯一索引,但由于null值不等于任一null值,因此能够成功插入
scott@ORCL> commit;
-->再次为表添加唯一复合索引,即基于id列与val列
scott@ORCL> create unique index i_t1_id_val on t1(id,val);
Index created.
-->插入null,'N'的记录时失败,提示违反唯一性约束
scott@ORCL> insert into t1 select null,'N' from dual;
insert into t1 select null,'N' from dual
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.I_T1_ID_VAL) violated
-->插入null,'Y'的记录时同样失败,提示违反唯一性约束
scott@ORCL> insert into t1 select null,'Y' from dual;
insert into t1 select null,'Y' from dual
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.I_T1_ID_VAL) violated
-->插入两个null值成功
scott@ORCL> insert into t1 select null,null from dual;
1 row created.
scott@ORCL> insert into t1 select null,null from dual;
1 row created.
scott@ORCL> insert into t1 select null,'A' from dual;
1 row created.
scott@ORCL> commit;
Commit complete.
scott@ORCL> set null unknown;
scott@ORCL> select * from t1;
ID VAL
---------- ------------------------------
unknown Y
unknown N
unknown unknown
unknown unknown
unknown A
scott@ORCL> exec dbms_stats.gather_table_stats('SCOTT','T1',cascade=>true);
scott@ORCL> select index_name,index_type,blevel,leaf_blocks,num_rows,status,distinct_keys
2 from user_indexes where table_name='T1';
INDEX_NAME INDEX_TYPE BLEVEL LEAF_BLOCKS NUM_ROWS STATUS DISTINCT_KEYS
--------------- ---------- ---------- ----------- ---------- -------- -------------
I_T1_ID NORMAL 0 0 0 VALID 0
I_T1_ID_VAL NORMAL 0 1 3 VALID 3
-->从上面的情形可知,
-->基于单列的唯一索引,可以多次插入null值,但其索引上并不存储null值。
-->基于多列的复合索引,尽管全为null值的行可以多次插入,但不全为null的重复行则不能被插入(注,非唯一复合索引不存在此限制,此处不演示)。
-->基于多列的复合索引,对于全为null值的索引值也不会被存储。如上面的情形,尽管插入了5条记录,复合索引中只存储了3条。
-->注:对于唯一性约束,null值不等于null值,同样(null,null)也不等同于(null,null),所以上面的两次null能够被插入。
二、null值与执行计划
scott@ORCL> set autot trace exp;
scott@ORCL> select * from t1 where id is null;
Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5 | 5 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T1 | 5 | 5 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("ID" IS NULL)
-->从上面的测试可知,由于null值是不被存储的,因此当使用id is null作为谓词时,走了全表扫描
scott@ORCL> select * from t1 where id is not null;
Execution Plan
----------------------------------------------------------
Plan hash value: 796913935
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1 | 0 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 1 | 0 (0)| 00:00:01 |
|* 2 | INDEX FULL SCAN | I_T1_ID | 1 | | 0 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID" IS NOT NULL)
-->从上面的测试可知,尽管当前表上id列上的所有值都为null,但不排除后续记录插入的id不为null的列。
-->故当使用id is not null作为谓词时,此时执行计划中走了索引全扫描。
-->下面来看看复合索引的情形
scott@ORCL> select * from t1 where val is null;
Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 2 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T1 | 2 | 2 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("VAL" IS NULL)
scott@ORCL> select * from t1 where val is not null;
Execution Plan
----------------------------------------------------------
Plan hash value: 1931510411
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 3 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | I_T1_ID_VAL | 3 | 3 | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("VAL" IS NOT NULL)
-->对于复合唯一索引的情形,当使用单列且非前导列谓词时,使用is null与 is not null等同于单列唯一索引的情形。
-->即原理也是一样的,val is null走全表扫描而val is not null走索引。因为null值不会被存储。
-->下面看看两个列都作为谓词的情形
scott@ORCL> select * from t1 where id is null and val is not null;
Execution Plan
----------------------------------------------------------
Plan hash value: 1040510552
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 3 | 1 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| I_T1_ID_VAL | 3 | 3 | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("ID" IS NULL)
filter("VAL" IS NOT NULL)
-->从上面的测试可知,尽管两个谓词列上都存在索引,一个为单列唯一索引,一个为复合唯一索引。Oracle 选择了复合索引I_T1_ID_VAL。
scott@ORCL> select * from t1 where id is not null and val is null;
Execution Plan
----------------------------------------------------------
Plan hash value: 796913935
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1 | 0 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 1 | 0 (0)| 00:00:01 |
|* 2 | INDEX FULL SCAN | I_T1_ID | 1 | | 0 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("VAL" IS NULL)
2 - filter("ID" IS NOT NULL)
-->同样的情形,谓词的顺序与复合索引定义的顺序一样,只不过第一个谓词为id is not null,而第二个谓词为val is null。
-->此时Oracle 选择了单列唯一索引I_T1_ID
-->看到此,不知道大家是否已明白,即哪个列为is not null,则会使用该列上的索引,原因还是那句话,索引不存储null值。
-->对于颠倒id列与val列以及id,val列为null或not null的其他不同组合情形不再演示,其执行计划类似。
三、使用is null走索引的情形
scott@ORCL> set autot off;
--删除原有表上的null值记录
scott@ORCL> delete from t1 where val not in('Y','N') or val is null;
3 rows deleted.
scott@ORCL> update t1 set id=1 where val='Y';
1 row updated.
scott@ORCL> update t1 set id=2 where val='N';
1 row updated.
scott@ORCL> commit;
Commit complete.
-->对原有记录更新后的情形
scott@ORCL> select * from t1;
ID VAL
---------- ------------------------------
1 Y
2 N
scott@ORCL> exec dbms_stats.gather_table_stats('SCOTT','T1',cascade=>true);
PL/SQL procedure successfully completed.
-->修改表列id使之具有not null约束的特性
scott@ORCL> alter table t1 modify(id not null);
Table altered.
scott@ORCL> set autot trace exp;
scott@ORCL> select * from t1 where id is null;
Execution Plan
----------------------------------------------------------
Plan hash value: 3160894736
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 0 (0)| |
|* 1 | FILTER | | | | | |
| 2 | INDEX FULL SCAN| I_T1_ID_VAL | 2 | 10 | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(NULL IS NOT NULL)
-->从上面的执行计划中可知,当表t1列id上具有not null 约束时,此时使用id is null选择了索引范围扫描
-->下面来看看列val is null 的情形
scott@ORCL> select * from t1 where val is null;
Execution Plan
----------------------------------------------------------
Plan hash value: 48744011
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 2 (0)| 00:00:01 |
|* 1 | INDEX FAST FULL SCAN| I_T1_ID_VAL | 1 | 5 | 2 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("VAL" IS NULL)
-->尽管val列上允许null值存在,但由于列id上具有not null 约束,且id列与val列存在复合唯一索引,因此此时选择了索引快速全扫描
-->其余不同组合情形大致相同,不再演示
-->为表t1新增一条val为null的记录
scott@ORCL> insert into t1 select 3,null from dual;
1 row created.
scott@ORCL> commit;
Commit complete.
scott@ORCL> exec dbms_stats.gather_table_stats('SCOTT','T1',cascade=>true);
PL/SQL procedure successfully completed.
-->下面的查询中可以看出尽管只有列id有not null约束,当所有的索引值都被存储
scott@ORCL> select index_name,index_type,blevel,leaf_blocks,num_rows,status,distinct_keys
2 from user_indexes where table_name='T1';
INDEX_NAME INDEX_TYPE BLEVEL LEAF_BLOCKS NUM_ROWS STATUS DISTINCT_KEYS
--------------- ---------- ---------- ----------- ---------- -------- -------------
I_T1_ID NORMAL 0 1 3 VALID 3
I_T1_ID_VAL NORMAL 0 1 3 VALID 3
-->Author : Robinson Cheng
-->Blog : http://blog.csdn.net/robinson_0612
四、总结 无论是单列唯一索引或复合唯一索引,对于可以为null的列或复合null值,Oracle不会为其存储索引值。 故在基于单列创建B树唯一索引或多列创建B树复合唯一索引的情形下, 当列上允许为null值时 where子句使用了基于is null的情形,其执行计划走全表扫描。 where子句使用了基于is not null的情形,其执行计划走索引扫描(索引范围扫描或索引全扫描)。 当列上不允许为null值时,存在非null约束 where子句使用了基于is null的情行,其执行计划走索引扫描。 where子句使用了基于is not null的情形,其执行计划也是走索引扫描。 注:此在Oracle 10g R2(linux)下的情形,不同的优化器版本可能会有偏差。
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