SQLSERVER SQL性能优化技巧

1.选择最有效率的表名顺序(只在基于规则的优化器中有效)

SQLSERVER的解析器按照从右到左的顺序处理FROM子句中的表名，因此FROM子句中写在最后的表（基础表driving table）将被最先处理，在FROM子句中包含多个表的情况下，必须选择记录条数最少的表作为基础表，当SQLSERVER处理多个表时，会运用排序及合并的方式连接它们，

首先，扫描第一个表（FROM子句中最后的那个表)并对记录进行排序；然后扫描第二个表（FROM子句中最后第二个表)；最后将所有从第二个表中检索出的记录与第一个表中合适记录进行合并
例如: 表 TAB1 16,384 条记录表 TAB2 5 条记录，选择TAB2作为基础表 (最好的方法) select count(*) from tab1,tab2 执行时间0.96秒，选择TAB2作为基础表 (不佳的方法) select count(*) from tab2,tab1 执行时间26.09秒；

如果有3个以上的表连接查询，那就需要选择交叉表（intersection table）作为基础表，交叉表是指那个被其他表所引用的表

例如:

EMP表描述了LOCATION表和CATEGORY表的交集

SELECT * FROM LOCATION L, CATEGORY C, EMP E WHERE E.EMP_NO BETWEEN 1000 AND 2000 AND E.CAT_NO = C.CAT_NO AND E.LOCN = L.LOCN

SELECT * FROM EMP E , LOCATION L , CATEGORY C WHERE E.CAT_NO = C.CAT_NO AND E.LOCN = L.LOCN AND E.EMP_NO BETWEEN 1000 AND 2000

SQLSERVER采用自下而上的顺序解析WHERE子句，根据这个原理，表之间的连接必须写在其他WHERE条件之前，那些可以过滤掉最大数量记录的条件必须写在WHERE子句的末尾
例如：
(低效,执行时间156.3秒)

SELECT * FROM EMP E WHERE SAL > 50000 AND JOB = 'MANAGER' AND 25 < (SELECT COUNT(*) FROM EMP WHERE MGR=E.EMPNO); (高效,执行时间10.6秒) SELECT * FROM EMP E WHERE 25 < (SELECT COUNT(*) FROM EMP WHERE MGR=E.EMPNO) AND SAL > 50000 AND JOB = 'MANAGER';

4.减少访问数据库的次数。当执行每条SQL语句时，SQLSERVER在内部执行了许多工作：解析SQL语句，估算索引的利用率，绑定变量，读数据块等等
由此可见，减少访问数据库的次数，就能实际上减少SQLSERVER的工作量，例如：
以下有三种方法可以检索出雇员号等于0342或0291的职员
方法1 (最低效)

SELECT EMP_NAME, SALARY, GRADE FROM EMP WHERE EMP_NO = 342;  SELECT EMP_NAME, SALARY, GRADE FROM EMP WHERE EMP_NO = 291;

DECLARE CURSOR C1 (E_NO NUMBER) IS SELECT EMP_NAME,SALARY,GRADE FROM EMP WHERE EMP_NO = E_NO; BEGIN OPEN C1(342); FETCH C1 INTO …,…,…; … OPEN C1(291); FETCH C1 INTO …,…,…; … CLOSE C1; END;

SELECT A.EMP_NAME, A.SALARY, A.GRADE, B.EMP_NAME, B.SALARY, B.GRADE FROM EMP A, EMP B WHERE A.EMP_NO = 342 AND B.EMP_NO = 291;

SELECT COUNT(*), SUM(SAL) FROM EMP WHERE DEPT_NO = '0020' AND ENAME LIKE 'SMITH%';  SELECT COUNT(*), SUM(SAL) FROM EMP WHERE DEPT_NO = '0030' AND ENAME LIKE 'SMITH%';

SELECT COUNT(DECODE(DEPT_NO, '0020', 'X', NULL)) D0020_COUNT, COUNT(DECODE(DEPT_NO, '0030', 'X', NULL)) D0030_COUNT, SUM(DECODE(DEPT_NO, '0020', SAL, NULL)) D0020_SAL, SUM(DECODE(DEPT_NO, 0030, SAL, NULL)) D0030_SAL FROM EMP WHERE ENAME LIKE 'SMITH%';

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

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)

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;

低效 SELECT * FROM EMP (基础表) WHERE EMPNO > 0 AND DEPTNO IN (SELECT DEPTNO FROM DEPT WHERE LOC = 'MELB')  高效 SELECT * FROM EMP (基础表) WHERE EMPNO > 0 AND EXISTS (SELECT 'X' FROM DEPT WHERE DEPT.DEPTNO = EMP.DEPTNO AND LOC = 'MELB')

SELECT … FROM EMP WHERE DEPT_NO NOT IN (SELECT DEPT_NO FROM DEPT WHERE DEPT_CAT = 'A');

高效 SELECT … FROM EMP A, DEPT B WHERE A.DEPT_NO = B.DEPT(+) AND B.DEPT_NO IS NULL AND B.DEPT_CAT(+) = 'A'  最高效 SELECT … FROM EMP E WHERE NOT EXISTS (SELECT 'X' FROM DEPT D WHERE D.DEPT_NO = E.DEPT_NO AND DEPT_CAT = 'A');

SELECT ENAME FROM EMP E WHERE EXISTS (SELECT 'X' FROM DEPT WHERE DEPT_NO = E.DEPT_NO AND DEPT_CAT = 'A');  更高效 SELECT ENAME FROM DEPT D, EMP E WHERE E.DEPT_NO = D.DEPT_NO AND DEPT_CAT = 'A';

低效 SELECT DISTINCT DEPT_NO, DEPT_NAME FROM DEPT D, EMP E WHERE D.DEPT_NO = E.DEPT_NO  高效 SELECT DEPT_NO, DEPT_NAME FROM DEPT D WHERE EXISTS (SELECT 'X' FROM EMP E WHERE E.DEPT_NO = D.DEPT_NO);

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