? ? ? ㈠ 先看兩個來自生產環境的真實案例：

? ? ? ? ?

? ? ? ? ?? 案例1

? ? ? ? ?? 案例2

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? ? ? ㈡ 原理：

? ? ?
? ? ?Oracle 在對于 varchar等字符型字段收集統計信息時，并不會對每個值都進行精確的統計
? ? ?而是，對值進行substr(,32)。一般來講，這種方式沒有什么問題
? ? ?但是，如果恰巧列中存儲的數據，前32bytes相同，那么，Oracle 的統計就會與實際情況不符

? ? ?
? ? ? ㈢ 測試：

?

?

    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


  

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? ? ? ㈣ 解決方案：

? ? ?
? ? ? ? ? 刪除索引列的直方圖
? ? ? ? ? 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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深入理解Oracle索引(10)：索引列字符類型統計信息的32位限制