统计、案例深入理解Oracle索引(10)：索引列字符类型统计信息的32位限制by小雨

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：

             

              案例1

              案例2

    

         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  aaaaaa46556
NAME  aaaaaa46734
NAME  aaaaaa46912
NAME  aaaaaa47090




hr@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  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa1
NAME  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa2
NAME  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa3
NAME  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa4




hr@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
----- --------------------------------------------------
ID
ID
NAME

         ㈣ 解决方案：

    
     
          删除引索列的直方图
          例子：

    

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