分类： Oracle
2012-11-29 09:32:10
SMON的作用还包括清理obj$数据字典基表(cleanup obj$)
OBJ$字典基表是Oracle Bootstarp启动自举的重要对象之一:
SQL> set linesize 80 ;
SQL> select sql_text from bootstrap$ where sql_text like 'CREATE TABLE OBJ$%';
SQL_TEXT
--------------------------------------------------------------------------------
CREATE TABLE OBJ$("OBJ#" NUMBER NOT NULL,"DATAOBJ#" NUMBER,"OWNER#" NUMBER NOT N
ULL,"NAME" VARCHAR2(30) NOT NULL,"NAMESPACE" NUMBER NOT NULL,"SUBNAME" VARCHAR2(
30),"TYPE#" NUMBER NOT NULL,"CTIME" DATE NOT NULL,"MTIME" DATE NOT NULL,"STIME"
DATE NOT NULL,"STATUS" NUMBER NOT NULL,"REMOTEOWNER" VARCHAR2(30),"LINKNAME" VAR
CHAR2(128),"FLAGS" NUMBER,"OID$" RAW(16),"SPARE1" NUMBER,"SPARE2" NUMBER,"SPARE3
" NUMBER,"SPARE4" VARCHAR2(1000),"SPARE5" VARCHAR2(1000),"SPARE6" DATE) PCTFREE
10 PCTUSED 40 INITRANS 1 MAXTRANS 255 STORAGE ( INITIAL 16K NEXT 1024K MINEXTEN
TS 1 MAXEXTENTS 2147483645 PCTINCREASE 0 OBJNO 18 EXTENTS (FILE 1 BLOCK 121))
触发场景
OBJ$基表是一张低级数据字典表，该表几乎对库中的每个对象(表、索引、包、视图等)都包含有一行记录。很多情况下，这些条目所代表的对象是不存在的对象(non-existent)，引起这种现象的一种可能的原因是对象本身已经被从数据库中删除了，但是对象条目仍被保留下来以满足消极依赖机制(negative dependency)。因为这些条目的存在会导致OBJ$表不断膨胀，这时就需要由SMON进程来删除这些不再需要的行。SMON会在实例启动(after startup of DB is started cleanup function again)时以及启动后的每12个小时执行一次清理任务(the cleanup is scheduled to run after startup and then every 12 hours)。
我们可以通过以下演示来了解SMON清理obj$的过程:
SQL> BEGIN
2 FOR i IN 1 .. 5000 LOOP
3 execute immediate ('create synonym gustav' || i || ' for
4 perfstat.sometable');
5 execute immediate ('drop synonym gustav' || i );
6 END LOOP;
7 END;
8 /
PL/SQL procedure successfully completed.
SQL> startup force;
ORACLE instance started.
Total System Global Area 1065353216 bytes
Fixed Size 2089336 bytes
Variable Size 486542984 bytes
Database Buffers 570425344 bytes
Redo Buffers 6295552 bytes
Database mounted.
Database opened.
SQL> select count(*) from user$ u, obj$ o
2 where u.user# (+)=o.owner# and o.type#=10 and not exists
3 (select p_obj# from dependency$ where p_obj# = o.obj#);
COUNT(*)
----------
5000
SQL> /
COUNT(*)
----------
5000
SQL> /
COUNT(*)
----------
4951
SQL> oradebug setospid 18457;
Oracle pid: 8, Unix process pid: 18457, image: oracle@rh2.oracle.com (SMON)
SQL> oradebug event 10046 trace name context forever ,level 1;
Statement processed.
SQL> oradebug tracefile_name;
/s01/admin/G10R2/bdump/g10r2_smon_18457.trc
select o.owner#,
o.obj#,
decode(o.linkname,
null,
decode(u.name, null, 'SYS', u.name),
o.remoteowner),
o.name,
o.linkname,
o.namespace,
o.subname
from user$ u, obj$ o
where u.use r#(+) = o.owner#
and o.type# = :1
and not exists
(select p_obj# from dependency$ where p_obj# = o.obj#)
order by o.obj#
for update
select null
from obj$
where obj# = :1
and type# = :2
and obj# not in
(select p_obj# from dependency$ where p_obj# = obj$.obj#)
delete from obj$ where obj# = :1
/* 删除过程其实较为复杂，可能要删除多个字典基表上的记录 */ ocp考试
