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昨天在书上看到SQL语句优化时,where条件顺序不同,性能不同,这个建议在Oracle11G版本还合适吗?方式1优于方式2?
方式1:
复制代码 代码如下:
select a.*
from students s,
class c
where
s.id = c.id
s.id = 'xxxxxxxx'
方式2:
复制代码 代码如下:
select a.*
from students s,
class c
where
s.id = 'xxxxxxxx'
s.id = c.id
10g中测试结果证明是一样的。
Microsoft Windows [版本 5.2.3790]
(C) 版权所有 1985-2003 Microsoft Corp.
C:\Documents and Settings\Administrator>sqlplus / as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on 星期六 5月 11 17:48:55 2013
Copyright (c) 1982, 2005, Oracle. All rights reserved.
连接到:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> alter system flush shared_pool;
系统已更改。
SQL> alter system flush buffer_cache;
系统已更改。
SQL> set autotrace on;
SQL> select *
2 from COUNTRIES c,
3 REGIONS r
4 where c.REGION_ID=r.REGION_ID and c.REGION_ID='4';
REGIONS r
*
第 3 行出现错误:
ORA-00942: 表或视图不存在
SQL> select *
2 from hr.COUNTRIES c,
3 hr. REGIONS r
4 where c.REGION_ID=r.REGION_ID and c.REGION_ID='4';
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
EG Egypt 4 4
Middle East and Africa
IL Israel 4 4
Middle East and Africa
KW Kuwait 4 4
Middle East and Africa
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
NG Nigeria 4 4
Middle East and Africa
ZM Zambia 4 4
Middle East and Africa
ZW Zimbabwe 4 4
Middle East and Africa
已选择6行。
执行计划
----------------------------------------------------------
Plan hash value: 4030513296
--------------------------------------------------------------------------------
----------------
| Id | Operation | Name | Rows | Bytes | Cost (%
CPU)| Time |
--------------------------------------------------------------------------------
----------------
| 0 | SELECT STATEMENT | | 6 | 168 | 2
(0)| 00:00:01 |
| 1 | NESTED LOOPS | | 6 | 168 | 2
(0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| REGIONS | 1 | 14 | 1
(0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | REG_ID_PK | 1 | | 0
(0)| 00:00:01 |
|* 4 | INDEX FULL SCAN | COUNTRY_C_ID_PK | 6 | 84 | 1
(0)| 00:00:01 |
--------------------------------------------------------------------------------
----------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("R"."REGION_ID"=4)
4 - filter("C"."REGION_ID"=4)
统计信息
----------------------------------------------------------
628 recursive calls
0 db block gets
127 consistent gets
20 physical reads
0 redo size
825 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
13 sorts (memory)
0 sorts (disk)
6 rows processed
SQL>
#############
SQL> alter system flush shared_pool;
系统已更改。
SQL> alter system flush buffer_cache;
系统已更改。
select *
from hr.COUNTRIES c,
hr. REGIONS r
where
c.REGION_ID='4'
6 and c.REGION_ID=r.REGION_ID;
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
EG Egypt 4 4
Middle East and Africa
IL Israel 4 4
Middle East and Africa
KW Kuwait 4 4
Middle East and Africa
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
NG Nigeria 4 4
Middle East and Africa
ZM Zambia 4 4
Middle East and Africa
ZW Zimbabwe 4 4
Middle East and Africa
已选择6行。
执行计划
----------------------------------------------------------
Plan hash value: 4030513296
--------------------------------------------------------------------------------
----------------
| Id | Operation | Name | Rows | Bytes | Cost (%
CPU)| Time |
--------------------------------------------------------------------------------
----------------
| 0 | SELECT STATEMENT | | 6 | 168 | 2
(0)| 00:00:01 |
| 1 | NESTED LOOPS | | 6 | 168 | 2
(0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| REGIONS | 1 | 14 | 1
(0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | REG_ID_PK | 1 | | 0
(0)| 00:00:01 |
|* 4 | INDEX FULL SCAN | COUNTRY_C_ID_PK | 6 | 84 | 1
(0)| 00:00:01 |
--------------------------------------------------------------------------------
----------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("R"."REGION_ID"=4)
4 - filter("C"."REGION_ID"=4)
统计信息
----------------------------------------------------------
656 recursive calls
0 db block gets
131 consistent gets
22 physical reads
0 redo size
825 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
13 sorts (memory)
0 sorts (disk)
6 rows processed
SQL>
方式1:
复制代码 代码如下:
select a.*
from students s,
class c
where
s.id = c.id
s.id = 'xxxxxxxx'
方式2:
复制代码 代码如下:
select a.*
from students s,
class c
where
s.id = 'xxxxxxxx'
s.id = c.id
10g中测试结果证明是一样的。
Microsoft Windows [版本 5.2.3790]
(C) 版权所有 1985-2003 Microsoft Corp.
C:\Documents and Settings\Administrator>sqlplus / as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on 星期六 5月 11 17:48:55 2013
Copyright (c) 1982, 2005, Oracle. All rights reserved.
连接到:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> alter system flush shared_pool;
系统已更改。
SQL> alter system flush buffer_cache;
系统已更改。
SQL> set autotrace on;
SQL> select *
2 from COUNTRIES c,
3 REGIONS r
4 where c.REGION_ID=r.REGION_ID and c.REGION_ID='4';
REGIONS r
*
第 3 行出现错误:
ORA-00942: 表或视图不存在
SQL> select *
2 from hr.COUNTRIES c,
3 hr. REGIONS r
4 where c.REGION_ID=r.REGION_ID and c.REGION_ID='4';
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
EG Egypt 4 4
Middle East and Africa
IL Israel 4 4
Middle East and Africa
KW Kuwait 4 4
Middle East and Africa
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
NG Nigeria 4 4
Middle East and Africa
ZM Zambia 4 4
Middle East and Africa
ZW Zimbabwe 4 4
Middle East and Africa
已选择6行。
执行计划
----------------------------------------------------------
Plan hash value: 4030513296
--------------------------------------------------------------------------------
----------------
| Id | Operation | Name | Rows | Bytes | Cost (%
CPU)| Time |
--------------------------------------------------------------------------------
----------------
| 0 | SELECT STATEMENT | | 6 | 168 | 2
(0)| 00:00:01 |
| 1 | NESTED LOOPS | | 6 | 168 | 2
(0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| REGIONS | 1 | 14 | 1
(0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | REG_ID_PK | 1 | | 0
(0)| 00:00:01 |
|* 4 | INDEX FULL SCAN | COUNTRY_C_ID_PK | 6 | 84 | 1
(0)| 00:00:01 |
--------------------------------------------------------------------------------
----------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("R"."REGION_ID"=4)
4 - filter("C"."REGION_ID"=4)
统计信息
----------------------------------------------------------
628 recursive calls
0 db block gets
127 consistent gets
20 physical reads
0 redo size
825 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
13 sorts (memory)
0 sorts (disk)
6 rows processed
SQL>
#############
SQL> alter system flush shared_pool;
系统已更改。
SQL> alter system flush buffer_cache;
系统已更改。
select *
from hr.COUNTRIES c,
hr. REGIONS r
where
c.REGION_ID='4'
6 and c.REGION_ID=r.REGION_ID;
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
EG Egypt 4 4
Middle East and Africa
IL Israel 4 4
Middle East and Africa
KW Kuwait 4 4
Middle East and Africa
CO COUNTRY_NAME REGION_ID REGION_ID
-- ---------------------------------------- ---------- ----------
REGION_NAME
-------------------------
NG Nigeria 4 4
Middle East and Africa
ZM Zambia 4 4
Middle East and Africa
ZW Zimbabwe 4 4
Middle East and Africa
已选择6行。
执行计划
----------------------------------------------------------
Plan hash value: 4030513296
--------------------------------------------------------------------------------
----------------
| Id | Operation | Name | Rows | Bytes | Cost (%
CPU)| Time |
--------------------------------------------------------------------------------
----------------
| 0 | SELECT STATEMENT | | 6 | 168 | 2
(0)| 00:00:01 |
| 1 | NESTED LOOPS | | 6 | 168 | 2
(0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| REGIONS | 1 | 14 | 1
(0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | REG_ID_PK | 1 | | 0
(0)| 00:00:01 |
|* 4 | INDEX FULL SCAN | COUNTRY_C_ID_PK | 6 | 84 | 1
(0)| 00:00:01 |
--------------------------------------------------------------------------------
----------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("R"."REGION_ID"=4)
4 - filter("C"."REGION_ID"=4)
统计信息
----------------------------------------------------------
656 recursive calls
0 db block gets
131 consistent gets
22 physical reads
0 redo size
825 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
13 sorts (memory)
0 sorts (disk)
6 rows processed
SQL>
标签:
where,条件
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暂无where条件顺序不同、性能不同示例探讨的评论...
稳了!魔兽国服回归的3条重磅消息!官宣时间再确认!
昨天有一位朋友在大神群里分享,自己亚服账号被封号之后居然弹出了国服的封号信息对话框。
这里面让他访问的是一个国服的战网网址,com.cn和后面的zh都非常明白地表明这就是国服战网。
而他在复制这个网址并且进行登录之后,确实是网易的网址,也就是我们熟悉的停服之后国服发布的暴雪游戏产品运营到期开放退款的说明。这是一件比较奇怪的事情,因为以前都没有出现这样的情况,现在突然提示跳转到国服战网的网址,是不是说明了简体中文客户端已经开始进行更新了呢?
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