维护col_usage$-曾充当过predicate

1. Set event 10052 at parameter file to disable cleanup of OBJ$ by SMON

EVENT="10052 trace name context forever, level 65535"

2. Startup database in restricted mode

3. Delete from OBJ$, COMMIT

SQL> delete from obj$ where (name,owner#) in ( select o.name,u.user# from user$ u, obj$ o
where u.user# (+)=o.owner# and o.type#=10 and not exists (select p_obj# from
dependency$ where p_obj# = o.obj#) );

SQL> commit;

SQL> Shutdown abort.

4. remove event 10052 from init.ora

5. Restart the database and monitor for the message in the ALERT LOG file
  
SMON的作用还包括维护col_usage$列监控统计信息基表。

最早在9i中引入了col_usage$字典基表，其目的在于监控column在SQL语句作为predicate的情况，col_usage$的出现完善了CBO中柱状图自动收集的机制。
create table col_usage$
(
  obj#              number,                                 /* object number */
  intcol#           number,                        /* internal column number */
  equality_preds    number,                           /* equality predicates */
  equijoin_preds    number,                           /* equijoin predicates */
  nonequijoin_preds number,                        /* nonequijoin predicates */
  range_preds       number,                              /* range predicates */
  like_preds        number,                         /* (not) like predicates */
  null_preds        number,                         /* (not) null predicates */
  timestamp         date      /* timestamp of last time this row was changed */
)
  storage (initial 200K next 100k maxextents unlimited pctincrease 0)
/
create unique index i_col_usage$ on col_usage$(obj#,intcol#)
  storage (maxextents unlimited)
/

在10g中我们默认使用’FOR ALL COLUMNS SIZE AUTO’的柱状图收集模式，而在9i中默认是’SIZE 1′即默认不收集柱状图，这导致许多9i中正常运行的应用程序在10g中CBO执行计划异常，详见;。’SIZE AUTO’意为由Oracle自动决定是否收集柱状图及柱状图的桶数，Oracle自行判断的依据就来源于col_usage$字典基表，若表上的某一列曾在硬解析(hard parse)过的SQL语句中充当过predicate(通俗的说就是where后的condition)的话，我们认为此列上有收集柱状图的必要，那么col_usage$上就会被加入该列曾充当predicate的记录。当DBMS_STATS.GATHER_TABLE_STATS存储过程以’SIZE AUTO’模式执行时，收集进程会检查col_usage$基表以判断哪些列之前曾充当过predicate，若充当过则说明该列有收集柱状图的价值。

SMON会每15分钟将shared pool中的predicate columns的数据刷新到col_usage$基表中(until periodically about every 15 minutes SMON flush the data into the data dictionary)，另外当instance shutdown时SMON会扫描col_usage$并找出已被drop表的相关predicate columns记录，并删除这部分”orphaned”孤儿记录。