oracle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)

 

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