Oracle体系结构学习笔记

Oracle体系结构由实例和一组数据文件组成，实例由SGA内存区，SGA意思是共享内存区，由share pool(共享池)、data buffer(数据缓冲区)、log buffer(日志缓冲区)组成

SGA内存区的share pool是解析SQL并保存执行计划的，然后SQL根据执行计划获取数据时先看data buffer里是否有数据，没数据才从磁盘读，然后还是读到data buffer里，下次就直接读data buffer的，当SQL更新时，data buffer的数据就必须写入磁盘备份，为了保护这些数据，才有log buffer，这就是大概的原理简介

系统结构关系图如图，图来自《收获，不止SQL优化》一书：

下面介绍共享池、数据缓冲、日志缓冲方面调优的例子

共享池相关例子

未使用使用绑定变量的情况，进行一下批量写数据，在登录系统，经常用的sql是select * from sys_users where username='admin'或者什么什么的，假如有很多用户登录，就需要执行很多次这样类似的sql，能不能用一条SQL代表？意思是不需要Oracle优化器每次都解析sql获取执行计划，对于这种类似的sql是没必要的，Oracle提供了绑定变量的方法，可以用于调优sql，然后一堆sql就可以用

select * from sys_users where username=:x

这里用一个变量x表示，具体例子如下，

新建一张表来测试

create table t (x int);

不使用绑定遍历，批量写数据

begin
	for i in 1 .. 1000
	loop
		execute immediate
		'insert into t values('|| i ||')';
		commit;
	end loop;
end;
/

输出

已用时间: 00: 00: 00.80

加上绑定遍历，绑定变量是用:x的形式

begin
	for i in 1 .. 100
	loop
		execute immediate
		'insert into t values( :x )' using i;
	commit;
	end loop;
end;
/

已用时间: 00: 00: 00.05

数据缓冲相关例子

这里介绍和数据缓存相关例子

(1) 清解析缓存

//创建一个表来测试
SQL> create table t as select * from dba_objects;
表已创建。
//设置打印行数
SQL> set linesize 1000
//设置执行计划开启
SQL> set autotrace on
//打印出时间
SQL> set timing on
//查询一下数据
SQL> select count(1) from t;

  COUNT(1)
----------
     72043

已用时间:  00: 00: 00.10

//清一下缓冲区缓存(ps:这个sql不能随便在生产环境执行)
SQL> alter system flush buffer_cache;
系统已更改。
已用时间:  00: 00: 00.08

//清一下共享池缓存(ps:这个sql不能随便在生产环境执行)
SQL> alter system flush shared_pool;

//再次查询，发现查询快了
SQL> select count(1) from t;

  COUNT(1)
----------
     72043

已用时间:  00: 00: 00.12

SQL>

日志缓冲相关例子

这里说明一下，日志关闭是可以提供性能的，不过在生生产环境还是不能随便用，只能说是一些特定创建，SQL如：

alter table [表名] nologging;

调优拓展知识

这些是看《收获，不止SQL优化》一书的小记

(1) 批量写数据事务问题

对于循环批量事务提交的问题，commit放在循环内和放在循环外的区别，

放在循环内，每次执行就提交一次事务，这种时间相对比较少的

begin
	for i in 1 .. 1000
	loop
		execute immediate
		'insert into t values('|| i ||')';
		commit;
	end loop;
end;

放在循环外，sql循环成功，再提交一次事务，这种时间相对比较多一点

begin
	for i in 1 .. 1000
	loop
		execute immediate
		'insert into t values('|| i ||')';
	end loop;
	commit;
end;

《收获，不止SQL优化》一书提供的脚本，用于查看逻辑读、解析、事务数等等情况：

select s.snap_date,
       decode(s.redosize, null, '--shutdown or end--', s.currtime) "TIME",
       to_char(round(s.seconds / 60, 2)) "elapse(min)",
       round(t.db_time / 1000000 / 60, 2) "DB time(min)",
       s.redosize redo,
       round(s.redosize / s.seconds, 2) "redo/s",
       s.logicalreads logical,
       round(s.logicalreads / s.seconds, 2) "logical/s",
       physicalreads physical,
       round(s.physicalreads / s.seconds, 2) "phy/s",
       s.executes execs,
       round(s.executes / s.seconds, 2) "execs/s",
       s.parse,
       round(s.parse / s.seconds, 2) "parse/s",
       s.hardparse,
       round(s.hardparse / s.seconds, 2) "hardparse/s",
       s.transactions trans,
       round(s.transactions / s.seconds, 2) "trans/s"
  from (select curr_redo - last_redo redosize,
               curr_logicalreads - last_logicalreads logicalreads,
               curr_physicalreads - last_physicalreads physicalreads,
               curr_executes - last_executes executes,
               curr_parse - last_parse parse,
               curr_hardparse - last_hardparse hardparse,
               curr_transactions - last_transactions transactions,
               round(((currtime + 0) - (lasttime + 0)) * 3600 * 24, 0) seconds,
               to_char(currtime, 'yy/mm/dd') snap_date,
               to_char(currtime, 'hh24:mi') currtime,
               currsnap_id endsnap_id,
               to_char(startup_time, 'yyyy-mm-dd hh24:mi:ss') startup_time
          from (select a.redo last_redo,
                       a.logicalreads last_logicalreads,
                       a.physicalreads last_physicalreads,
                       a.executes last_executes,
                       a.parse last_parse,
                       a.hardparse last_hardparse,
                       a.transactions last_transactions,
                       lead(a.redo, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_redo,
                       lead(a.logicalreads, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_logicalreads,
                       lead(a.physicalreads, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_physicalreads,
                       lead(a.executes, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_executes,
                       lead(a.parse, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_parse,
                       lead(a.hardparse, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_hardparse,
                       lead(a.transactions, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_transactions,
                       b.end_interval_time lasttime,
                       lead(b.end_interval_time, 1, null) over(partition by b.startup_time order by b.end_interval_time) currtime,
                       lead(b.snap_id, 1, null) over(partition by b.startup_time order by b.end_interval_time) currsnap_id,
                       b.startup_time
                  from (select snap_id,
                               dbid,
                               instance_number,
                               sum(decode(stat_name, 'redo size', value, 0)) redo,
                               sum(decode(stat_name,
                                          'session logical reads',
                                          value,
                                          0)) logicalreads,
                               sum(decode(stat_name,
                                          'physical reads',
                                          value,
                                          0)) physicalreads,
                               sum(decode(stat_name, 'execute count', value, 0)) executes,
                               sum(decode(stat_name,
                                          'parse count (total)',
                                          value,
                                          0)) parse,
                               sum(decode(stat_name,
                                          'parse count (hard)',
                                          value,
                                          0)) hardparse,
                               sum(decode(stat_name,
                                          'user rollbacks',
                                          value,
                                          'user commits',
                                          value,
                                          0)) transactions
                          from dba_hist_sysstat
                         where stat_name in
                               ('redo size',
                                'session logical reads',
                                'physical reads',
                                'execute count',
                                'user rollbacks',
                                'user commits',
                                'parse count (hard)',
                                'parse count (total)')
                         group by snap_id, dbid, instance_number) a,
                       dba_hist_snapshot b
                 where a.snap_id = b.snap_id
                   and a.dbid = b.dbid
                   and a.instance_number = b.instance_number
                 order by end_interval_time)) s,
       (select lead(a.value, 1, null) over(partition by b.startup_time order by b.end_interval_time) - a.value db_time,
               lead(b.snap_id, 1, null) over(partition by b.startup_time order by b.end_interval_time) endsnap_id
          from dba_hist_sys_time_model a, dba_hist_snapshot b
         where a.snap_id = b.snap_id
           and a.dbid = b.dbid
           and a.instance_number = b.instance_number
           and a.stat_name = 'DB time') t
 where s.endsnap_id = t.endsnap_id
 order by s.snap_date, time desc;