彻底搞懂oracle的标量子查询

oracle标量子查询和自定义函数有时用起来比较方便，而且开发人员也经常使用，数据量小还无所谓，数据量大，往往存在性能问题。

以下测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.

由上面的执行计划可以知道，b表执行2次，返回2行

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.

由上面的执行计划可以知道，b表执行3次，返回2行

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.

由上面的执行计划可以知道，b表执行9次，返回2行

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.

由上面的执行计划可以知道，b表执行2次，返回2行

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.

b.name字段全部为‘b1’，由上面的执行计划可以知道，b表执行9次，返回9行

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>

关联字段a.id全部为1，a表有9行，标量子查询相当于执行9次select name from b where b.id=1 ，oracle也不傻，starts=1，说明只执行了1次。

总结：

理想状态下，a.id为主键，没有重复值，那么a表返回多少行，b表就要被执行多少次。

特殊情况下，a.id的distinct值只有n个，那么b表只执行n次。

彻底搞懂oracle的标量子查询,布布扣,bubuko.com

时间： 2024-12-23 12:16:51

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