㈠ 先看两个来自生产环境的真实案例：

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;

/