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  • 深入理解Oracle索引(10):索引列字符类型统计信息的32位限制

         ㈠ 先看两个来自生产环境的真实案例:

             

              案例1

              案例2

     

         ㈡ 原理:


         
         Oracle 在对于 varchar等字符型字段收集统计信息时,并不会对每个值都进行精确的统计
         而是,对值进行substr(,32)。一般来讲,这种方式没有什么问题
         但是,如果恰巧列中存储的数据,前32bytes相同,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:


         
              删除索引列的直方图
              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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  • 原文地址:https://www.cnblogs.com/javawebsoa/p/3027222.html
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