关于DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC的一些发现

任务在哪

这个是11g以后的自动收集统计信息的后台任务，10g之前是在dba_scheduler_jobs里查看

SQL> SELECT CLIENT_NAME               ,STATUS                    ,MEAN_INCOMING_TASKS_7_DAYS,MEAN_INCOMING_TASKS_30_DAYS
FROM   DBA_AUTOTASK_CLIENT
WHERE  CLIENT_NAME = 'auto optimizer stats collection'
/  2    3    4    5    6    7  CLIENT_NAME                                                      STATUS
---------------------------------------------------------------- --------
MEAN_INCOMING_TASKS_7_DAYS MEAN_INCOMING_TASKS_30_DAYS
-------------------------- ---------------------------
auto optimizer stats collection                                  ENABLED2                  2.25806452

可以在dba_autotask_job_history中看到历史执行情况

什么原理

根据1592404.1，后台的GATHER_DATABASE_STATS_JOB_PROC就是使用gather auto的option的情况。

根据1233203.1

How does auto optimizer stats collection prioritize which tables are analyzed first?

Accurate statistics are important on all objects. The GATHER_DATABASE_STATS_JOB_PROC procedure called by the 'auto optimizer stats collection' job prioritizes database objects that have no statistics. This means that objects that most need statistics are processed first. Once these are done then objects with stale statistics are addressed. For these, there is no particular prioritization. The statistics may be ordered in some way but it is cursory, ordering by owner,object_name,part_name just to be consistent. 

基本是先收集empty的，再收集stale的，剩下的按用户，对象名，分区名等。

观测手段

可以用以下过程查看下一次gather auto的列表

SQL> 
set line 150
set serveroutput on
declareobj_auto dbms_stats.ObjectTab;
begindbms_stats.gather_database_stats(options => 'LIST AUTO',objlist => obj_auto);for i in 1..obj_auto.countloopdbms_output.put_line('Auto list---> Owner:  '||obj_auto(i).ownname||'   Object name:  '||obj_auto(i).objName||'- -Object type:  '||obj_auto(i).objType);end loop;  
end;
/