oracle标量子查询和自己定义函数有时用起来比較方便，并且开发者也常常使用。数据量小还无所谓。数据量大，往往存在性能问题。

下面測试帮助大家彻底搞懂标量子查询。

SQL> create table a (id int,name varchar2(10));

Table created.

SQL> create table b (id int,name varchar2(10));

Table created.

SQL> insert into a values (1,'a1');

1 row created.

SQL> insert into a values (2,'a2');

1 row created.

SQL> insert into b values (1,'b1');

1 row created.

SQL> insert into b values (2,'b2');

1 row created.

SQL> commit;

Commit complete.

SQL> @getlvall

Session altered.

SQL> select a.*,(select name from b where b.id=a.id) from a;

        ID NAME                 (SELECTNAMEFROMBWHER

---------- -------------------- --------------------

         1 a1                   b1

         2 a2                   b2

SQL> @getplanspe

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  8rv825dykpx1m, child number 0

-------------------------------------

select a.*,(select name from b where b.id=a.id) from a

Plan hash value: 2657529235

------------------------------------------------------------------------------------

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

------------------------------------------------------------------------------------

|*  1 |  TABLE ACCESS FULL| B    |      2 |      1 |      2 |00:00:00.01 |      14 |

|   2 |  TABLE ACCESS FULL| A    |      1 |      2 |      2 |00:00:00.01 |       8 |

------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - filter("B"."ID"=:B1)

Note

-----

   - dynamic sampling used for this statement

22 rows selected.
<strong>--由上面的运行计划能够知道，b表运行2次。返回2行</strong>
SQL> insert into a values (3,'a3');

1 row created.

SQL> commit;

Commit complete.

SQL> select a.*,(select name from b where b.id=a.id) from a;

        ID NAME                 (SELECTNAMEFROMBWHER

---------- -------------------- --------------------

         1 a1                   b1

         2 a2                   b2

         3 a3

SQL> @getplanspe

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  8rv825dykpx1m, child number 0

-------------------------------------

select a.*,(select name from b where b.id=a.id) from a

Plan hash value: 2657529235

------------------------------------------------------------------------------------

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

------------------------------------------------------------------------------------

|*  1 |  TABLE ACCESS FULL| B    |      3 |      1 |      2 |00:00:00.01 |      21 |

|   2 |  TABLE ACCESS FULL| A    |      1 |      2 |      3 |00:00:00.01 |       8 |

------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - filter("B"."ID"=:B1)

Note

-----

   - dynamic sampling used for this statement

22 rows selected.
<strong>--由上面的运行计划能够知道，b表运行3次。返回2行</strong>
SQL> insert into a values (4,'a4');

1 row created.

SQL> insert into a values (5,'a5');

1 row created.

SQL> insert into a values (6,'a6');

1 row created.

SQL> insert into a values (7,'a7');

1 row created.

SQL> insert into a values (8,'a8');

1 row created.

SQL> insert into a values (9,'a9');

1 row created.

SQL> commit;

Commit complete.

SQL> select a.*,(select name from b where b.id=a.id) from a;

        ID NAME                 (SELECTNAMEFROMBWHER

---------- -------------------- --------------------

         1 a1                   b1

         2 a2                   b2

         3 a3

         4 a4

         5 a5

         6 a6

         7 a7

         8 a8

         9 a9

9 rows selected.

SQL> @getplanspe

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  8rv825dykpx1m, child number 0

-------------------------------------

select a.*,(select name from b where b.id=a.id) from a

Plan hash value: 2657529235

------------------------------------------------------------------------------------

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

------------------------------------------------------------------------------------

|*  1 |  TABLE ACCESS FULL| B    |      9 |      1 |      2 |00:00:00.01 |      63 |

|   2 |  TABLE ACCESS FULL| A    |      1 |      2 |      9 |00:00:00.01 |       8 |

------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - filter("B"."ID"=:B1)

Note

-----

   - dynamic sampling used for this statement

22 rows selected.
<strong>

--由上面的运行计划能够知道，b表运行9次，返回2行</strong>
SQL> update b set name='b1';

2 rows updated.

SQL> commit;

Commit complete.

SQL> select a.*,(select name from b where b.id=a.id) from a;

        ID NAME                 (SELECTNAMEFROMBWHER

---------- -------------------- --------------------

         1 a1                   b1

         2 a2                   b1

         3 a3

         4 a4

         5 a5

         6 a6

         7 a7

         8 a8

         9 a9

9 rows selected.

SQL> @getplanspe

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  8rv825dykpx1m, child number 0

-------------------------------------

select a.*,(select name from b where b.id=a.id) from a

Plan hash value: 2657529235

------------------------------------------------------------------------------------

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

------------------------------------------------------------------------------------

|*  1 |  TABLE ACCESS FULL| B    |      9 |      1 |      2 |00:00:00.01 |      63 |

|   2 |  TABLE ACCESS FULL| A    |      1 |      2 |      9 |00:00:00.01 |       8 |

------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - filter("B"."ID"=:B1)

Note

-----

   - dynamic sampling used for this statement

22 rows selected.
<strong>--由上面的运行计划能够知道，b表运行2次，返回2行</strong>
SQL> insert into b values (3,'b1');

1 row created.

SQL> insert into b values (4,'b1');

1 row created.

SQL> insert into b values (5,'b1');

1 row created.

insert into b values (6,'b1');b1');

1 row created.

SQL> insert into b values (7,'b1');

1 row created.

SQL> insert into b values (8,'b1');

1 row created.

SQL> insert into b values (9,'b1');

1 row created.

SQL> commit;

Commit complete.

SQL> select a.*,(select name from b where b.id=a.id) from a;

        ID NAME                 (SELECTNAMEFROMBWHER

---------- -------------------- --------------------

         1 a1                   b1

         2 a2                   b1

         3 a3                   b1

         4 a4                   b1

         5 a5                   b1

         6 a6                   b1

         7 a7                   b1

         8 a8                   b1

         9 a9                   b1

9 rows selected.

SQL> @getplanspe

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  8rv825dykpx1m, child number 0

-------------------------------------

select a.*,(select name from b where b.id=a.id) from a

Plan hash value: 2657529235

------------------------------------------------------------------------------------

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

------------------------------------------------------------------------------------

|*  1 |  TABLE ACCESS FULL| B    |      9 |      1 |      9 |00:00:00.01 |      63 |

|   2 |  TABLE ACCESS FULL| A    |      1 |      2 |      9 |00:00:00.01 |       8 |

------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - filter("B"."ID"=:B1)

Note

-----

   - dynamic sampling used for this statement

22 rows selected.
<strong>--b.name字段所有为‘b1’。由上面的运行计划能够知道。b表运行9次。返回9行</strong>
SQL> update a set id=1;

9 rows updated.

SQL> commit;

Commit complete.

SQL> select * from a;

        ID NAME

---------- --------------------

         1 a1

         1 a2

         1 a3

         1 a4

         1 a5

         1 a6

         1 a7

         1 a8

         1 a9

9 rows selected.

SQL> select * from b;

        ID NAME

---------- --------------------

         1 b1

         2 b1

         3 b1

         4 b1

         5 b1

         6 b1

         7 b1

         8 b1

         9 b1

9 rows selected.

SQL> select a.*,(select name from b where b.id=a.id) from a;

        ID NAME                 (SELECTNAMEFROMBWHER

---------- -------------------- --------------------

         1 a1                   b1

         1 a2                   b1

         1 a3                   b1

         1 a4                   b1

         1 a5                   b1

         1 a6                   b1

         1 a7                   b1

         1 a8                   b1

         1 a9                   b1

9 rows selected.

SQL> @getplanspe

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  8rv825dykpx1m, child number 0

-------------------------------------

select a.*,(select name from b where b.id=a.id) from a

Plan hash value: 2657529235

------------------------------------------------------------------------------------

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

------------------------------------------------------------------------------------

|*  1 |  TABLE ACCESS FULL| B    |      1 |      1 |      1 |00:00:00.01 |       7 |

|   2 |  TABLE ACCESS FULL| A    |      1 |      2 |      9 |00:00:00.01 |       8 |

------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - filter("B"."ID"=:B1)

Note

-----

   - dynamic sampling used for this statement

22 rows selected.

SQL> 
</pre><pre name="code" class="sql"><strong><span style="font-size:18px;">关联字段a.id所有为1。a表有9行，标量子查询相当于运行9次select name from b where b.id=1 ，oracle也不傻，starts=1。说明仅仅运行了1次。

总结：

理想状态下，a.id为主键，没有反复值。那么a表返回多少行，b表就要被运行多少次。

特殊情况下，a.id的distinct值仅仅有n个，那么b表仅仅运行n次。

</span></strong>

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