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月归档:七月 2011
select max(id),min(id) from table优化
1、查看数据库版本
SQL> select * from v$version where rownum<2;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
2、创建表和索引
create table t_a as select * from dba_objects;
create index t_a_ind on t_a(object_id);
3、查询最大值
SQL> select max(object_id) from t_a;
执行计划
———————————————————-
Plan hash value: 3226265922
————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FULL SCAN (MIN/MAX)| T_A_IND | 1 | 13 | 2 (0)| 00:00:01 |
————————————————————————————–
Note—— dynamic sampling used for this statement (level=2)
统计信息
———————————————————-
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
431 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
4、查询最小值
SQL> select min(object_id) from t_a;
执行计划
———————————————————-
Plan hash value: 3226265922
————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | |
|
| 2 | INDEX FULL SCAN (MIN/MAX)| T_A_IND | 1 | 13 | 2 (0)| 00:00:01 |
————————————————————————————–
Note—— dynamic sampling used for this statement (level=2)
统计信息
———————————————————-
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
429 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
5、查询最大值和最小值
SQL> select max(object_id),min(object_id) from t_a;
执行计划
———————————————————-
Plan hash value: 2127980459
—————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————
| 0 | SELECT STATEMENT | | 1 | 13 | 293 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| T_A | 78093 | 991K| 293 (1)| 00:00:04 |
—————————————————————————
Note—— dynamic sampling used for this statement (level=2)
统计信息
———————————————————-
4 recursive calls
0 db block gets
1119 consistent gets
1044 physical reads
0 redo size
502 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
6、查询最大值和最小值(排除null)
SQL> select max(object_id),min(object_id) from t_a where object_id is not null;
执行计划
———————————————————-
Plan hash value: 1214261695
———————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|
———————————————————————————
| 0 | SELECT STATEMENT | | 1 | 13 | 50 (2)| 00:00:01
|
| 1 | SORT AGGREGATE | | 1 | 13 | |
|
|* 2 | INDEX FAST FULL SCAN| T_A_IND | 78093 | 991K| 50 (2)| 00:00:01
|
———————————————————————————
Predicate Information (identified by operation id):
—————————————————
2 – filter(“OBJECT_ID” IS NOT NULL)
Note—— dynamic sampling used for this statement (level=2)
统计信息
———————————————————-
4 recursive calls
0 db block gets
242 consistent gets
0 physical reads
0 redo size
502 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
7、分别查询最大值和最小值
SQL> select (select max(object_id) from t_a) max,(select min(object_id) from t_a) min from dual;
执行计划
———————————————————-
Plan hash value: 312201770
————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | | 2 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | |
| 2 | INDEX FULL SCAN (MIN/MAX)| T_A_IND | 1 | 13 | 2 (0)| 00:00:01 |
| 3 | SORT AGGREGATE | | 1 | 13 | |
| 4 | INDEX FULL SCAN (MIN/MAX)| T_A_IND | 1 | 13 | 2 (0)| 00:00:01 |
| 5 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |
————————————————————————————–
Note—— dynamic sampling used for this statement (level=2)
统计信息
———————————————————-
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
480 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
说明:
1、单查询最大值或者最小值,sql会自动走index
2、如果同时查询最大值和最小值,sql会使用全表扫描,而不是我们想象的索引快速扫描
3、加上where 排除掉null的情况,sql使用索引快速扫描,原因是:在不能确定索引列不为null(或者没有排除掉null)的情况下,不会使用索引快速扫描,而sql为了保证正确而采用了全表扫描
4、INDEX FULL SCAN (MIN/MAX)扫描效率很搞,所以把最大值,最小值分开查询,提高执行效率
5、其他写法
SQL> select (select /*+ index_asc(t_a t_a_ind) */ object_id from t_a where rownu
m=1) min ,(select /*+ index_desc(t_a t_a_ind) */ object_id from t_a where rownum=1)
max from dual;
执行计划
———————————————————-
Plan hash value: 674626822
—————————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————————
| 0 | SELECT STATEMENT | | 1 | | 2 (0)| 00:
00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | INDEX FULL SCAN | T_A_IND | 78093 | 991K| 2 (0)| 00:
00:01 |
|* 3 | COUNT STOPKEY | | | | |
| 4 | INDEX FULL SCAN DESCENDING| T_A_IND | 78093 | 991K| 2 (0)| 00:
00:01 |
| 5 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |
—————————————————————————————
Predicate Information (identified by operation id):
—————————————————
1 – filter(ROWNUM=1)
3 – filter(ROWNUM=1)
Note—— dynamic sampling used for this statement (level=2)
统计信息
———————————————————-
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
480 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
注意:
1)这个要正确执行,需要一个前提条件object_id这列要为not null限制条件,不然会hint提示无效
2)从执行计划的统计信息上看,这个和INDEX FULL SCAN (MIN/MAX)方式的执行效率一样
发表在 Oracle性能优化
一条评论
statspack安装
今天发现关于statspack的安装没有记录下来,现在记录下,下次需要用的时候直接查找blog,而不用在硬盘中查找相关安装配置文件
1、创建statspack专用表空间
CREATE TABLESPACE SP
DATAFILE
‘/opt/oracle/oradata/usercent/PS_1.001.dbf’ SIZE 10M AUTOEXTEND ON NEXT 10M MAXSIZE 5G,
‘/opt/oracle/oradata/usercent/PS_1.002.dbf’ SIZE 10M AUTOEXTEND ON NEXT 10M MAXSIZE 5G,
‘/opt/oracle/oradata/usercent/PS_1.003.dbf’ SIZE 10M AUTOEXTEND ON NEXT 10M MAXSIZE 5G
LOGGING
PERMANENT
EXTENT MANAGEMENT LOCAL AUTOALLOCATE
BLOCKSIZE 8K
SEGMENT SPACE MANAGEMENT AUTO;
2、检测系统中是否有PERFSTAT用户
如果有,需要查询所跑的业务,做一些处理,然后删除
select * from dba_users where username=’PERFSTAT’;
drop user PERFSTAT cascade;
3、检查job_queue_processes
SQL> show parameter job
NAME TYPE VALUE
———————————— ———– ——————————
job_queue_processes integer 6
如果为0,请使用
alter system set job_queue_processes=6;
进行修改,当然数值不一定是6
4、检查timed_statistics
SQL> show parameter timed_statistics
NAME TYPE VALUE
———————————— ———– ——————————
timed_statistics boolean TRUE
如果timed_statistics为false,请使用下面命令修改为true
alter system set timed_statistics=true;
4、安装statspack
4.1)使用sys用户登录数据库
4.2)安装statspack主体程序
执行@?/rdbms/admin/spcreate.sql
如果失败,请执行@?/rdbms/admin/spdrop.sql,然后重新安装
4.3)设置自动采集信息
执行@?/rdbms/admin/spauto.sql
可以通过vi修改采集时间频率
发表在 Oracle性能优化
一条评论
GREATEST/LEAST函数
GREATEST
greatest( expr1, expr2, … expr_n )
expr1, expr2, . expr_n 可以是值也可以是函数.
函数功能: 取得值最大值
SQL> SELECT GREATEST(10,21,3,15,18) FROM dual;
GREATEST(10,21,3,15,18)
———————–
21
SQL> SELECT GREATEST(‘abc’,’bca’,’cba’,’bac’,’cabb’,’cbaa’) FROM dual;
GREATEST(‘ABC’,’BCA’,’CBA’,’BA
——————————
cbaa
LEAST
least( expr1, expr2, … expr_n )
expr1, expr2, . expr_n 可以是值也可以是函数.
函数功能: 取得值最小值
SQL> SELECT least(10,21,3,15,18) FROM dual;
LEAST(10,21,3,15,18)
——————–
3
SQL> SELECT least(‘abc’,’bca’,’cba’,’a’,’bac’,’cabb’,’cbaa’) FROM dual;
LEAST(‘ABC’,’BCA’,’CBA’,’A’,’B
——————————
a
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