降低逻辑读优化SQL（转）

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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=RULE1  0   FILTER2  1     TABLE ACCESS (FULL) OF 'SYS_USER'3  1     INDEX (UNIQUE SCAN) OF 'PK_SYS_GRP' (UNIQUE)Statistics----------------------------------------------------------14  recursive calls4  db block gets30590 consistent gets0  physical reads0  redo size1723  bytes sent via SQL*Net to client425  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  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 allselect * 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 processedExecution 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unionall是不需要排序的

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