[索引相关] 不使用索引的情况 - 4

-- 数据类型的隐式转换，导致索引失效
-- 应该把转换写在等式右边

go@XE> create table t (x char(1) constraint t_pk primary key,y date);

Table created.

go@XE> desc t
 Name                  Null?    Type
 --------------------- -------- ----------------------------------------
 X                              NOT NULL CHAR(1)
 Y                              DATE

go@XE> insert into t values ('5',sysdate);

1 row created.

go@XE> commit;

Commit complete.

go@XE> set autotrace traceonly explain
go@XE> select * from t where x=5;

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    12 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |     1 |    12 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(TO_NUMBER("X")=5) -- 列X隐式转换

Note
-----
   - dynamic sampling used for this statement

-- 强制使用索引，导致索引全扫描，而不是唯一扫描
go@XE> select /*+ index(t,t_pk) */* from t where x=5;

Execution Plan
----------------------------------------------------------
Plan hash value: 1399892806

------------------------------------------------------------------------------------
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |     1 |    12 |    34   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T    |     1 |    12 |    34   (0)| 00:00:01 |
|*  2 |   INDEX FULL SCAN           | T_PK |     1 |       |    26   (0)| 00:00:01 |
------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter(TO_NUMBER("X")=5)

Note
-----
   - dynamic sampling used for this statement

-- 正确的写法
go@XE> select * from t where x='5';

Execution Plan
----------------------------------------------------------
Plan hash value: 1303508680

------------------------------------------------------------------------------------
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |     1 |    12 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T    |     1 |    12 |     1   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | T_PK |     1 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("X"='5')