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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;
