彻底搞懂oracle的标量子查询
2014-07-14 15:59
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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.
<strong>--由上面的执行计划可以知道,b表执行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>--由上面的执行计划可以知道,b表执行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> --由上面的执行计划可以知道,b表执行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>--由上面的执行计划可以知道,b表执行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 cr ca32 eated. 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</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。</span></strong>
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