oracle索引

唯一索引：unique
对于唯一索引来说，由于null不等于null，所以就算一列中有多个null ，还是可以建立唯一所有的。但是这样不能用到索引，因为有null值。

但是对添加唯一复合索引时，是不能插入null的。

 

-->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