[Oracle]高效的PL/SQL程序设计(一)--伪列ROWNUM使用技巧

--1. 取前10行


select * from hr.employees where rownum<=10

--2. 按照first_name升序，取前10位
--正确方法 oracle处理机制: --> hr.employees全表扫描
--> SORT ORDER BY STOPKEY 只排序前10行，作为一个矩阵结构
-->剩下的行与第10行进行比较，合适的进入矩阵,否则抛弃
--优点：RAM中少量排序，速度快(不需要在内存或者temp表空间进行全表排序), 并不真正排序整个结果集，但概念上做了整个结果集的排序
--注意第一,二个rownum的区别


select rownum,t.* from (select rownum,employees.* from hr.employees order by first_name) t where rownum<=10

--执行计划


SELECT STATEMENT, GOAL = CHOOSE Cost=5 Cardinality=10 Bytes=15622


COUNT STOPKEY


VIEW Object owner=SCOTT Cost=5 Cardinality=107 Bytes=15622


SORT ORDER BY STOPKEY Cost=5 Cardinality=107 Bytes=7276


COUNT


TABLE ACCESS FULL Object owner=HR Object name=EMPLOYEES Cost=2 Cardinality=107 Bytes=7276

--返回的结果


ROWNUM ROWNUM EMPLOYEE_ID FIRST_NAME LAST_NAME EMAIL


1 22 121 Adam Fripp AFRIPP


2 97 196 Alana Walsh AWALSH


3 48 147 Alberto Errazuriz AERRAZUR


4 4 103 Alexander Hunold AHUNOLD


5 16 115 Alexander Khoo AKHOO


6 86 185 Alexis Bull ABULL


7 59 158 Allan McEwen AMCEWEN


8 76 175 Alyssa Hutton AHUTTON


9 68 167 Amit Banda ABANDA


10 88 187 Anthony Cabrio ACABRIO

--3.取第11-20行
--推荐方法 COUNT STOPKEY --> 当查询到20行时，执行了查询中止的命令 Cardinality=20

select t.* from (select /*+ First_rows */ rownum rnum,employees.* from hr.employees where rownum<=20) t where rnum>10


SELECT STATEMENT, GOAL = CHOOSE Cost=2 Cardinality=20 Bytes=2920


VIEW Object owner=SCOTT Cost=2 Cardinality=20 Bytes=2920


COUNT STOPKEY


TABLE ACCESS FULL Object owner=HR Object name=EMPLOYEES Cost=2 Cardinality=107 Bytes=7276

--不推荐方法 COUNT --> 建立了整个结果集 Cardinality=107


select t.* from (select rownum rnum,employees.* from hr.employees) t where rnum between 11 and 20




SELECT STATEMENT, GOAL = CHOOSE Cost=2 Cardinality=107 Bytes=15622


VIEW Object owner=SCOTT Cost=2 Cardinality=107 Bytes=15622


COUNT


TABLE ACCESS FULL Object owner=HR Object name=EMPLOYEES Cost=2 Cardinality=107 Bytes=7276

-4. -当使用内嵌视图时, ORACLE优化程序可能会整合视图, 看下面那句sql语句的explain


select * from(


select employees.* from hr.employees


) emp,


(select departments.* from hr.departments


) dept


where emp.department_id=dept.department_id


SELECT STATEMENT, GOAL = CHOOSE Cost=5 Cardinality=106 Bytes=9328


HASH JOIN Cost=5 Cardinality=106 Bytes=9328


TABLE ACCESS FULL Object owner=HR Object name=DEPARTMENTS Cost=2 Cardinality=27 Bytes=540


TABLE ACCESS FULL Object owner=HR Object name=EMPLOYEES Cost=2 Cardinality=107 Bytes=7276

--如果觉得ORACLE所走的access path不是你想要或者说执行时间过长, 可以尝试在内嵌视图中增加rownum, 这个时候内嵌视图会被实体化(当作一个实体表), 这种情况下ORACLE优化程序无法整合视图! 可能会带来性能上的提升@!


select * from(


select employees.*,rownum from hr.employees


) emp,


(select departments.*,rownum from hr.departments


) dept


where emp.department_id=dept.department_id


SELECT STATEMENT, GOAL = CHOOSE Cost=5 Cardinality=106 Bytes=22790


HASH JOIN Cost=5 Cardinality=106 Bytes=22790


VIEW Object owner=TEST Cost=2 Cardinality=27 Bytes=1863


COUNT


TABLE ACCESS FULL Object owner=HR Object name=DEPARTMENTS Cost=2 Cardinality=27 Bytes=540


VIEW Object owner=TEST Cost=2 Cardinality=107 Bytes=15622


COUNT


TABLE ACCESS FULL Object owner=HR Object name=EMPLOYEES Cost=2 Cardinality=107 Bytes=7276