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NOT < to >;= NOT <= to >; 译者按:
在这个例子中,作者犯了一些错误. 例子中的低效率SQL是不能被执行的. 我做了一些测试:
SQL>; select * from emp where NOT empno >; 1; no rows selected Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP' 2 1 INDEX (RANGE SCAN) OF 'EMPNO' (UNIQUE)
SQL>; select * from emp where empno <= 1; no rows selected Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP' 2 1 INDEX (RANGE SCAN) OF 'EMPNO' (UNIQUE)
两者的效率完全一样，也许这符合作者关于” 在某些时候, ORACLE优化器会自动将NOT转化成相对应的关系操作符” 的观点
-------------------------------------------------------------------------------- txfy 回复于：2003-12-02 10:08:15
35. 用>;=替代>;
如果DEPTNO上有一个索引, 高效:
SELECT * FROM EMP
WHERE DEPTNO >;=4 低效:
SELECT *
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FROM EMP
WHERE DEPTNO >;3
两者的区别在于, 前者DBMS将直接跳到第一个DEPT等于4的记录而后者将首先定位到DEPTNO=3的记录并且向前扫描到第一个DEPT大于3的记录. 36. 用UNION替换OR (适用于索引列)
通常情况下, 用UNION替换WHERE子句中的OR将会起到较好的效果. 对索引列使用OR将造成全表扫描. 注意, 以上规则只针对多个索引列有效. 如果有column没有被索引, 查询效率可能会因为你没有选择OR而降低. 在下面的例子中, LOC_ID 和REGION上都建有索引. 高效:
SELECT LOC_ID , LOC_DESC , REGION FROM LOCATION WHERE LOC_ID = 10 UNION
SELECT LOC_ID , LOC_DESC , REGION FROM LOCATION
WHERE REGION = “MELBOURNE” 低效:
SELECT LOC_ID , LOC_DESC , REGION FROM LOCATION
WHERE LOC_ID = 10 OR REGION = “MELBOURNE”
如果你坚持要用OR, 那就需要返回记录最少的索引列写在最前面. 注意:
WHERE KEY1 = 10 (返回最少记录) OR KEY2 = 20 (返回最多记录)
ORACLE 内部将以上转换为 WHERE KEY1 = 10 AND
((NOT KEY1 = 10) AND KEY2 = 20) 译者按:
下面的测试数据仅供参考: (a = 1003 返回一条记录 , b = 1 返回1003条记录) SQL>; select * from unionvsor /*1st test*/ 2 where a = 1003 or b = 1; 1003 rows selected. Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 CONCATENATION
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2 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 3 2 INDEX (RANGE SCAN) OF 'UB' (NON-UNIQUE) 4 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 5 4 INDEX (RANGE SCAN) OF 'UA' (NON-UNIQUE) Statistics
---------------------------------------------------------- 0 recursive calls 0 db block gets 144 consistent gets 0 physical reads 0 redo size
63749 bytes sent via SQL*Net to client 7751 bytes received via SQL*Net from client 68 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1003 rows processed
SQL>; select * from unionvsor /*2nd test*/ 2 where b = 1 or a = 1003 ; 1003 rows selected. Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 CONCATENATION
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 3 2 INDEX (RANGE SCAN) OF 'UA' (NON-UNIQUE) 4 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 5 4 INDEX (RANGE SCAN) OF 'UB' (NON-UNIQUE) Statistics
---------------------------------------------------------- 0 recursive calls 0 db block gets 143 consistent gets 0 physical reads 0 redo size
63749 bytes sent via SQL*Net to client 7751 bytes received via SQL*Net from client 68 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1003 rows processed
SQL>; select * from unionvsor /*3rd test*/ 2 where a = 1003
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3 union
4 select * from unionvsor 5 where b = 1; 1003 rows selected. Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 SORT (UNIQUE) 2 1 UNION-ALL
3 2 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 4 3 INDEX (RANGE SCAN) OF 'UA' (NON-UNIQUE) 5 2 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR' 6 5 INDEX (RANGE SCAN) OF 'UB' (NON-UNIQUE) Statistics
---------------------------------------------------------- 0 recursive calls 0 db block gets 10 consistent gets 0 physical reads 0 redo size
63735 bytes sent via SQL*Net to client 7751 bytes received via SQL*Net from client 68 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 1003 rows processed
用UNION的效果可以从consistent gets和 SQL*NET的数据交换量的减少看出
37. 用IN来替换OR
下面的查询可以被更有效率的语句替换: 低效:
SELECT…. FROM LOCATION WHERE LOC_ID = 10 OR LOC_ID = 20 OR LOC_ID = 30 高效 SELECT…
FROM LOCATION
WHERE LOC_IN IN (10,20,30);
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译者按:
这是一条简单易记的规则，但是实际的执行效果还须检验，在ORACLE8i下，两者的执行路径似乎是相同的．
38. 避免在索引列上使用IS NULL和IS NOT NULL
避免在索引中使用任何可以为空的列，ORACLE将无法使用该索引 ．对于单列索引，如果列包含空值，索引中将不存在此记录. 对于复合索引，如果每个列都为空，索引中同样不存在此记录. 如果至少有一个列不为空，则记录存在于索引中． 举例:
如果唯一性索引建立在表的A列和B列上, 并且表中存在一条记录的A,B值为(123,null) , ORACLE将不接受下一条具有相同A,B值（123,null）的记录(插入). 然而如果
所有的索引列都为空，ORACLE将认为整个键值为空而空不等于空. 因此你可以插入1000 条具有相同键值的记录,当然它们都是空!
因为空值不存在于索引列中,所以WHERE子句中对索引列进行空值比较将使ORACLE停用该索引. 举例:
低效: (索引失效) SELECT …
FROM DEPARTMENT
WHERE DEPT_CODE IS NOT NULL;
高效: (索引有效) SELECT …
FROM DEPARTMENT WHERE DEPT_CODE >;=0; 39. 总是使用索引的第一个列
如果索引是建立在多个列上, 只有在它的第一个列(leading column)被where子句引用时,优化器才会选择使用该索引. 译者按:
这也是一条简单而重要的规则. 见以下实例.
SQL>; create table multiindexusage ( inda number , indb number , descr varchar2(10)); Table created.
SQL>; create index multindex on multiindexusage(inda,indb); Index created.
SQL>; set autotrace traceonly
SQL>; select * from multiindexusage where inda = 1; Execution Plan
---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE