使用or展开来改写SQL查询

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