如何正确的使用or展开来改写SQL查询

来源：赛迪网    作者：Alizze

【赛迪网－IT技术报道】问题:

下面的这条sql应该怎么优化?

select * from sys_user 
where user_code = 'zhangyong' 
or user_code in 
(select grp_code 
from sys_grp 
where sys_grp.user_code = 'zhangyong')


Execution Plan
----------------------------------------------------------
0  SELECT STATEMENT Optimizer=RULE
1  0   FILTER
2  1     TABLE ACCESS (FULL) OF 'SYS_USER'
3  1     INDEX (UNIQUE SCAN) OF 'PK_SYS_GRP' (UNIQUE)


Statistics
----------------------------------------------------------
14  recursive calls
4  db block gets
30590 consistent gets
0  physical reads
0  redo size
1723  bytes sent via SQL*Net to client
425  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
3  rows processed

里面的查询返回的记录数一般只有一两条，但sys_user表的数据很多，怎么样才能让这条sql以sys_grp为驱动表？

表中记录情况如下:

SQL> select count(*) from sys_grp;
COUNT(*)----------25130
SQL> select count(*) from sys_user;
COUNT(*)
----------
15190

优化:

降低逻辑读是优化SQL的基本原则之一

我们尝试通过降低逻辑读来加快SQL的执行.

这里我们使用or展开来改写SQL查询:

select * from sys_user where user_code = 'zhangyong' 
union all
select * from sys_user where user_code <> 'zhangyong' 
and user_code in (select grp_code from sys_grp 
where sys_grp.user_code = 'zhangyong')

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         130 consistent gets
          0  physical reads
          0  redo size
       1723  bytes sent via SQL*Net to client
        425  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          3  rows processed

Execution Plan
----------------------------------------------------------
0      SELECT STATEMENT Optimizer=RULE
1    0   UNION-ALL
2    1     TABLE ACCESS (BY INDEX ROWID) OF 'SYS_USER'
3    2       INDEX (UNIQUE SCAN) OF 'PK_SYS_USER' (UNIQUE)
4    1     NESTED LOOPS
5    4       VIEW OF 'VW_NSO_1'
6    5         SORT (UNIQUE)
7    6           TABLE ACCESS (BY INDEX ROWID) OF 'SYS_GRP'
8    7             INDEX (RANGE SCAN) OF 'FK_SYS_USER_CODE' (NON-UNIQUE)
9    4       TABLE ACCESS (BY INDEX ROWID) OF 'SYS_USER'
10    9         INDEX (UNIQUE SCAN) OF 'PK_SYS_USER' (UNIQUE)

我们注意到,通过改写,逻辑读减少到130，从30590到130这是一个巨大的提高,减少逻辑读最终会减少资源消耗,提高SQL的执行效率.

这个改写把Filter改为了Nest LOOP,索引得以充分利用.从而大大提高了性能.

我们同时注意到，这里引入了一个排序

排序来自于这一步:

----------------------------------
6  5  SORT (UNIQUE)
7  6    TABLE ACCESS (BY INDEX ROWID) OF 'SYS_GRP'
8  7   INDEX (RANGE SCAN) OF 'FK_SYS_USER_CODE' (NON-UNIQUE)

----------------------------------

注释：在'SYS_GRP'表中，user_code 是非唯一键值，在in值判断里，要做sort unique排序，去除重复值，这里的union all是不需要排序的。