ORACLE优化器 _数据库技巧

避免使用HAVING子句, HAVING 只会在检索出所有记录之后才对结果集进行过滤. 这个处理需要排序,总计等操作. 如果能通过WHERE子句限制记录的数目,那就能减少这方面的开销.

例如:

低效:

SELECT REGION，AVG(LOG_SIZE)

FROM LOCATION

GROUP BY REGION

HAVING REGION REGION != ‘SYDNEY’

AND REGION != ‘PERTH’

高效

SELECT REGION，AVG(LOG_SIZE)

FROM LOCATION

WHERE REGION REGION != ‘SYDNEY’

AND REGION != ‘PERTH’

GROUP BY REGION

(译者按: HAVING 中的条件一般用于对一些集合函数的比较,如COUNT() 等等. 除此而外,一般的条件应该写在WHERE子句中)

15. 减少对表的查询

在含有子查询的SQL语句中,要特别注意减少对表的查询.

例如:

低效

SELECT TAB_NAME

FROM TABLES

WHERE TAB_NAME = ( SELECT TAB_NAME

FROM TAB_COLUMNS

WHERE VERSION = 604)

AND DB_VER= ( SELECT DB_VER

FROM TAB_COLUMNS

WHERE VERSION = 604)

高效

SELECT TAB_NAME

FROM TABLES

WHERE (TAB_NAME,DB_VER)

= ( SELECT TAB_NAME,DB_VER)

FROM TAB_COLUMNS

WHERE VERSION = 604)

Update 多个Column 例子:

低效:

UPDATE EMP

SET EMP_CAT = (SELECT MAX(CATEGORY) FROM EMP_CATEGORIES),

SAL_RANGE = (SELECT MAX(SAL_RANGE) FROM EMP_CATEGORIES)

WHERE EMP_DEPT = 0020;

高效:

UPDATE EMP

SET (EMP_CAT, SAL_RANGE)

= (SELECT MAX(CATEGORY) , MAX(SAL_RANGE)

FROM EMP_CATEGORIES)

WHERE EMP_DEPT = 0020;

16. 通过内部函数提高SQL效率.

SELECT H.EMPNO,E.ENAME,H.HIST_TYPE,T.TYPE_DESC,COUNT(*)

FROM HISTORY_TYPE T,EMP E,EMP_HISTORY H

WHERE H.EMPNO = E.EMPNO