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.
