PL/SQL中使用or展开进行sql调整

王朝mssql·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

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

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

0SELECT STATEMENT Optimizer=RULE

10 FILTER

21 TABLE Access (FULL) OF 'SYS_USER'

31 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

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

0recursive calls

0db block gets

130 consistent gets

0physical reads

0redo size

1723bytes sent via SQL*Net to client

425bytes received via SQL*Net from client

2SQL*Net roundtrips to/from client

1sorts (memory)

0sorts (disk)

3rows processed

Execution Plan

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

0SELECT STATEMENT Optimizer=RULE

10 UNION-ALL

21 TABLE ACCESS (BY INDEX ROWID) OF 'SYS_USER'

32 INDEX (UNIQUE SCAN) OF 'PK_SYS_USER' (UNIQUE)

41 NESTED LOOPS

54 VIEW OF 'VW_NSO_1'

65 SORT (UNIQUE)

76 TABLE ACCESS (BY INDEX ROWID) OF 'SYS_GRP'

87 INDEX (RANGE SCAN) OF 'FK_SYS_USER_CODE' (NON-UNIQUE)

94 TABLE ACCESS (BY INDEX ROWID) OF 'SYS_USER'

109 INDEX (UNIQUE SCAN) OF 'PK_SYS_USER' (UNIQUE)

我们注重到,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.

我们同时注重到,这里引入了一个排序

排序来自于这一步:

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

65SORT (UNIQUE)

76TABLE ACCESS (BY INDEX ROWID) OF 'SYS_GRP'

87 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值判定里,要做sort unique排序,去除重复值

这里的union all是不需要排序的

 
 
 
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