最近几天都在学习OCP相关考试题目，以后陆续把自己做错或者感觉特别经典的题目记录下来。
    其实之前没有想到数据库还可以关闭特定表的信息自动收集，直到看了以下这道题才知道。
    然后去看了下资料如何关闭：
    You have statistics collected for some selected tables. Your requirement is that the statistics for the
tables and all dependent indexes must not be overwritten by further statistics collection until a certain
point of time.
How would you achieve this?
A.Lock statistics for the tables.
B.Change STALE_PERCENT to zero for the tables.
C.Set the TIMED_STATISTICS parameter to TRUE.
D.Set the STATISTICS_LEVEL parameter to BASIC.
E.Set the OPTIMIZER_USE_PENDING parameter statistics to TRUE.
简单说为了让特定表的统计信息不被覆盖，可以把这些表的信息收集关闭掉：
 BEGIN
   DBMS_STATS.lock_table_stats(USER,'STAT');
   END;
其中stat为表名，相应的重新开启
 BEGIN
   DBMS_STATS.unlock_table_stats(USER,'STAT');
   END;

进一步，也可以对一个模式进行开关，工具包：

Statistics for a table or schema can be locked. After statistics are locked, you can make no modifications to the statistics until the statistics have been unlocked. Locking procedures are useful in a static environment in which you want to guarantee that the statistics never change.

The DBMS_STATS package provides two procedures for locking (LOCK_SCHEMA_STATS and LOCK_TABLE_STATS) and two procedures for unlocking statistics (UNLOCK_SCHEMA_STATS and UNLOCK_TABLE_STATS).

另外，手动收集信息：
BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(OWNNAME          => USER,
                                TABNAME          => 'STAT',
                                ESTIMATE_PERCENT => 100,
                                METHOD_OPT       => 'for all columns size skewonly',
                                CASCADE          => TRUE);
END;
查询统计结果:
SELECT NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, CHAIN_CNT, AVG_ROW_LEN
  FROM USER_TAB_STATISTICS
 WHERE TABLE_NAME = 'STAT';