使用or展开进行sql优化（即sql语法union all代替or可以提高效率）

问题:

这样一条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)

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

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