java - Oracle DUAL 表返回 "no rows selected"

Question

我收到这个查询:

insert into biller_onboarding_tbl values(BILLER_ONBOARDING_ID_SEQ.NEXTVAL,'1','blah','j', '1','dsad','das','dasd','dsad','dasd','dasd','dsadsa')

1 rows inserted.

select * from BILLER_ONBOARDING_TBL;

返回所有行 -

select BILLER_ONBOARDING_ID_SEQ.NEXTVAL from dual;

no rows selected

为什么会发生这种情况？

Answer 1

select BILLER_ONBOARDING_ID_SEQ.NEXTVAL from dual;

双表可能为空。凭借 SYSDB 权限，您可以删除/截断SYS.DUAL 表。

但是，即使 Dual 不返回任何行，您仍然会获得 sequence.nextval 值。 优化器在某种程度上肯定会始终从双表中获取行。

SQL> conn sys@pdborcl as sysdba
Enter password:
Connected.
SQL> show user
USER is "SYS"
SQL> select * from dual;

D
-
X

SQL> delete from dual;

1 row deleted.

SQL> commit;

Commit complete.

SQL> select * from dual;

no rows selected

因此，双表中没有行。

让我们创建一个序列并进行测试:

SQL> create sequence s;

Sequence created.

SQL> select s.nextval from dual;

   NEXTVAL
----------
         1

SQL> select * from dual;

no rows selected

SQL> select s.nextval from dual;

   NEXTVAL
----------
         2

解释计划显示FAST DUAL操作仍然获取一行:

SQL> explain plan for select s.nextval from dual;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------
Plan hash value: 3499163060

-----------------------------------------------------------------
| Id  | Operation        | Name | Rows  | Cost (%CPU)| Time     |
-----------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     1 |     2   (0)| 00:00:01 |
|   1 |  SEQUENCE        | S    |       |            |          |
|   2 |   FAST DUAL      |      |     1 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------

9 rows selected.

SQL>

注意:DUAL是一个特殊的表，属于SYS。它是数据字典的一部分。操作数据字典并不是一个好主意。不推荐。

如果双表尚未被操作，那么您应该能够从 dba_objects 看到以下输出:

SQL> column owner format a15
SQL> column object_name format a15
SQL> column object_type format a15
SQL> SELECT owner, object_name, object_type FROM dba_objects WHERE object_name='DUAL';

OWNER           OBJECT_NAME     OBJECT_TYPE
--------------- --------------- ---------------
SYS             DUAL            TABLE
PUBLIC          DUAL            SYNONYM

SQL>

在您的情况下，可能已经创建了一个名为 DUAL 的自定义表，并且该表不包含任何行。

如果是这种情况，并且 SYS.DUAL 尚未被操作，则尝试使用:

select BILLER_ONBOARDING_ID_SEQ.NEXTVAL from SYS.dual;