试试去搞懂oracle的标量子查询

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.