Oracle索引欄位發生隱式轉換仍然能夠使用索引

前言

最近在優化SQL過程中，發現執行計劃中，某張表使用了索引但是根據執行計劃謂詞資訊部分，發現該索引被隱式轉換了。

在我以前的認知中，索引欄位被隱式轉換後是用不了索引的，因此自己構造如下實驗做測試。

環境構造

14:43:47 SYS@zkm(17)> create user zkm identified by oracle;                              

User created.

Elapsed: 00:00:00.05
14:43:54 SYS@zkm(17)> grant dba to zkm;

 
Grant succeeded.

Elapsed: 00:00:00.01
14:43:59 SYS@zkm(17)> create table zkm.t ( n1 char(20),n2 char(20),n3 char(30));

Table created.

Elapsed: 00:00:00.10
14:44:07 SYS@zkm(17)> insert into zkm.t values('1','1','1');

1 row created.

Elapsed: 00:00:00.01
14:44:10 SYS@zkm(17)> insert into zkm.t values('2','2','2');

 
1 row created.

Elapsed: 00:00:00.00
14:44:14 SYS@zkm(17)> commit;

Commit complete.

Elapsed: 00:00:00.00
14:44:16 SYS@zkm(17)> create index zkm.idx on zkm.t(n1);

Index created.

Elapsed: 00:00:00.05
14:44:21 SYS@zkm(17)> analyze table zkm.t compute statistics;

Table analyzed.

Elapsed: 00:00:00.11

過程

14:48:45 SYS@zkm
 
(17)>  conn zkm/oracle
Connected.
14:48:52 ZKM@zkm(17)> set line 500
14:48:57 ZKM@zkm(17)> set autot traceo exp
14:49:03 ZKM@zkm(17)> select /*+ index(t IDX) */ * from zkm.t where n1=1;
Elapsed: 00:00:00.00

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

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    70 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |     1 |    70 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter(TO_NUMBER("N1")=1)

14:49:11 ZKM@zkm(17)> select /*+ index(t IDX) */ * from zkm.t where n1='1';
Elapsed: 00:00:00.00

Execution Plan
----------------------------------------------------------
Plan hash value: 2770274160

------------------------------------------------------------------------------------
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |     1 |    70 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T    |     1 |    70 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX  |     1 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------

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

   2 - access("N1"='1')

這個時候發現並不能模擬出想要的結果。

當n1=1的時候，根據謂詞資訊可以得知，此時n1被做了隱式轉換。

於此同時，並沒法使用索引了。

研究了很久還是甚覺不解，於是將生產遇到的問題在群裡問問看大神們。

於是，根據下表，hint換成index_ffs再試試看。

發光時代