問題:
這樣一條sql應該怎么優化?
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
里面的查詢返回的記錄數一般只有一兩條，但sys_user表的數據很多，怎么樣才能讓這條sql以sys_grp為驅動表？
表中記錄情況如下:
SQL> select count(*) from sys_grp;
COUNT(*)
----------
25130
SQL> select count(*) from sys_user;
COUNT(*)
----------
15190
優化:
降低邏輯讀是優化SQL的基本原則之一
我們嘗試通過降低邏輯讀來加快SQL的執行.
這里我們使用or展開來改寫SQL查詢:
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)
我們注意到,通過改寫,邏輯讀減少到130，從30590到130這是一個巨大的提高,減少邏輯讀最終會減少資源消耗,提高SQL的執行效率.這個改寫把Filter改為了Nest LOOP,索引得以充分利用.從而大大提高了性能.
我們同時注意到，這里引入了一個排序
排序來自於這一步:
-----------------------------------------------------------------------------------------
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 是非唯一鍵值
在in值判斷里，要做sort unique排序，去除重復值
這里的union all是不需要排序的