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
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
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);
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;
|
本类阅读TOP10