Oracle段收缩功能

1. 了解段收缩

2. 自动执行Segment Advisor

2.1 在OEM查看并设置

服务器 -> Oracle Scheduler -> 自动维护任务 -> 配置，来启用或禁用“段指导”任务，同时这里还可以设置“优化程序统计信息搜集”，“自动 SQL 优化”

2.2 使用SQL命令设置

--禁用自动执行Segment Advisor
BEGIN
dbms_auto_task_admin.disable(client_name => 'auto space advisor', operation => NULL, window_name => NULL);
END;
/

--启用自动执行Segment Advisor
BEGIN
dbms_auto_task_admin.enable(client_name => 'auto space advisor', operation => NULL, window_name => NULL);
END;
/

3. 收缩段

收缩段核心步骤：

--启用行移动
alter table T3 enable row movement;
--与大多数Oracle SQL命令不同，添加COMPACT关键字后，实际执行的操作不多反少，即HWM不移动
alter table T3 shrink space compact;
--去掉COMPACT关键字后，执行HWM移动
alter table T3 shrink space;
--禁用行移动
alter table T3 disable row movement;

实验：执行段分析和收缩操作

3.1 创建表T3

--创建表T3
create table T3 (c1 varchar2(4000));

3.2 往T3插入1000行（使用大值），然后使用小值更新这些行

--往T3插入1000行（使用大值）
begin
  for i in 1..1000 loop
    insert into T3 select rpad(i,3900,'*') from dual;
  end loop;
  commit; 
end;
/
--T3大小
select segment_name, BYTES/1024/1024 "MB" from user_segments where segment_name = 'T3';
SEGMENT_NAME                           MB
------------------------------ ----------
T3                                      8
--使用小值更新这些行
update t3 set c1='1';
commit;
--此时再次查询表T3大小是不变的

3.3 设置启动Segment Advisor作业

variable task_id number
declare
  name varchar2(100);
  descr varchar2(500);
  obj_id number;
begin
  name := ''; --unique name
  descr := 'Check T3 table';
  dbms_advisor.create_task('Segment Advisor', :task_id, name, descr, null);
  dbms_advisor.create_object(name, 'TABLE', 'JINGYU', 'T3', null, null, obj_id);
  dbms_advisor.set_task_parameter(name, 'RECOMMEND_ALL', 'TRUE');
  dbms_advisor.execute_task(name);
end;
/

执行过程如下：

SQL> conn jingyu/jingyu
Connected.
SQL> variable task_id number
SQL> declare
  2    name varchar2(100);
  3    descr varchar2(500);
  4    obj_id number;
  5  begin
  6    name := ''; --unique name
  7    descr := 'Check T3 table';
  8    dbms_advisor.create_task('Segment Advisor', :task_id, name, descr, null);
  9    dbms_advisor.create_object(name, 'TABLE', 'JINGYU', 'T3', null, null, obj_id);
 10    dbms_advisor.set_task_parameter(name, 'RECOMMEND_ALL', 'TRUE');
 11    dbms_advisor.execute_task(name);
 12  end;
 13  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:02.01
SQL> 
SQL> 
SQL> print task_id

   TASK_ID
----------
      1358

根据上面的TASK_ID查询DBA_ADVISOR_FINDINGS

col owner for a20
col task_name for a20
col type for a20
select owner, task_id, task_name, type, message, more_info from dba_advisor_findings where task_id=1358;
OWNER                   TASK_ID TASK_NAME            TYPE
-------------------- ---------- -------------------- --------------------
MESSAGE
------------------------------------------------------------------------------------------------------------------------
MORE_INFO
------------------------------------------------------------------------------------------------------------------------
JINGYU                     1358 TASK_1358            PROBLEM
The free space in the object is less than 10MB.
Allocated Space:8388608: Used Space:954000: Reclaimable Space :7434608:

3.4 执行段收缩

alter table T3 enable row movement;
alter table T3 shrink space compact;
alter table T3 shrink space;
alter table T3 disable row movement;
--T3大小
select segment_name, BYTES/1024/1024 "MB" from user_segments where segment_name = 'T3';
SEGMENT_NAME                           MB
------------------------------ ----------
T3                                  .0625

可以看到，T3表由8M大小成功收缩成0.0625M大小。至此，完成T3表的shrink操作。