上班之余抽点时间出来写写博文，希望对新接触的朋友有帮助。今天在这里和大家一起学习一下统计、案例-

         ㈠ 先看两个来自产生境环的实在案例：

             

              

              

    

         ㈡ 道理：

    

     

     Oracle 在对于 varchar等字符型字段集收统计信息时，其实不会对每一个值都停止确精的统计

     而是，对值停止substr(,32)。一般来讲，种这方法没有什么问题

     但是，如果恰巧列中存储的据数，前32bytes同相，那么，Oracle 的统计就会与实际情况不符

    

     

     ㈢ 测试：

    

hr@ORCL> drop table t purge;Table dropped.hr@ORCL> create table t (id number,name varchar2(300));Table created.hr@ORCL> create index idx_t on t (name);Index created.hr@ORCL> insert into t select rownum,lpad('a',6,'a')||to_char(rownum) from dba_objects;50322 rows created.hr@ORCL> commit;Commit complete.hr@ORCL> exec dbms_stats.gather_table_stats(user,'T',null,null,method_opt=>'for columns size 254 name',cascade=>true);PL/SQL procedure successfully completed.hr@ORCL> select column_name, endpoint_actual_value  2        from user_tab_histograms  3       where table_name = 'T'  4             and rownum<5  5        order by column_name, endpoint_Number;COLUM ENDPOINT_ACTUAL_VALUE----- --------------------------------------------------NAME  aaaaaa46556NAME  aaaaaa46734NAME  aaaaaa46912NAME  aaaaaa47090hr@ORCL> truncate table t;Table truncated.hr@ORCL> insert into t select rownum,lpad('a',31,'a')||to_char(rownum) from dba_objects;50322 rows created.hr@ORCL> exec dbms_stats.gather_table_stats(user,'T',null,null,method_opt=>'for columns size 254 name',cascade=>true);PL/SQL procedure successfully completed.hr@ORCL> select column_name, endpoint_actual_value  2        from user_tab_histograms  3       where table_name = 'T'  4             and rownum<5  5        order by column_name, endpoint_Number;COLUM ENDPOINT_ACTUAL_VALUE----- --------------------------------------------------NAME  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa1NAME  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa2NAME  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa3NAME  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa4hr@ORCL> truncate table t;Table truncated.hr@ORCL> insert into t select rownum,lpad('a',32,'a')||to_char(rownum) from dba_objects;50322 rows created.hr@ORCL> exec dbms_stats.gather_table_stats(user,'T',null,null,method_opt=>'for columns size 254 name',cascade=>true);PL/SQL procedure successfully completed.hr@ORCL> select column_name, endpoint_actual_value  2        from user_tab_histograms  3       where table_name = 'T'  4             and rownum<5  5        order by column_name, endpoint_Number;COLUM ENDPOINT_ACTUAL_VALUE----- --------------------------------------------------IDIDNAME

         ㈣ 解决方案：

    

     

          删除引索列的直方图

          例子：

    

SQL> begindbms_stats.gather_table_stats(ownname => 'HR',                              tabname => 'T' ,                              estimate_percent => null ,                              method_opt => 'for columns SIZE 1 name' ,                              cascade => true);end;/

文章结束给大家分享下程序员的一些笑话语录： 很多所谓的牛人也不过如此，离开了你，微软还是微软，Google还是Google，苹果还是苹果，暴雪还是暴雪，而这些牛人离开了公司，自己什么都不是。