本文只涉及使用dbm_stats来收集或删除列的统计信息的一些命令，以备查询。

测试表如下(实验环境为10.2.0.4):

SQL> create table test(i int,a varchar2(30));  Table created.    SQL> insert into test select rownum,object_name from all_objects;  9907 rows created.  

简单的说，列的统计信息，主要包括两种类型：

• 只有基本信息：收集的统计信息只有1个桶(bucket)
• 包含柱状图信息：收集的统计信息包含2到254个桶

也就是说，如果想收集列的基本信息，同时不希望收集柱状图，则需要指定bucket的size为1：

SQL> exec dbms_stats.gather_table_stats(user, 'TEST',  cascade=>false, method_opt=>'for columns i size 1');    PL/SQL procedure successfully completed.                                                                                 SQL> select column_name,num_distinct,num_buckets,low_value,high_value,density,avg_col_len,histogram    2  from user_tab_columns where table_name='TEST';    COLUMN_NAM NUM_DISTINCT NUM_BUCKETS LOW_VALUE  HIGH_VALUE    DENSITY AVG_COL_LEN HISTOGRAM  ---------- ------------ ----------- ---------- ---------- ---------- ----------- ---------------  I                  9907           1 C102       C26408     .000100939           4 NONE  A                                                                                NONE  

如果要收集列的柱状图信息，则bucket的个数必须大于等于2(最多不超过254)

SQL> exec dbms_stats.gather_table_stats(user, 'TEST',  cascade=>false, method_opt=>'for columns i size 2');    PL/SQL procedure successfully completed.    SQL> select column_name,num_distinct,num_buckets,low_value,high_value,density,avg_col_len,histogram    2  from user_tab_columns where table_name='TEST';    COLUMN_NAM NUM_DISTINCT NUM_BUCKETS LOW_VALUE  HIGH_VALUE    DENSITY AVG_COL_LEN HISTOGRAM  ---------- ------------ ----------- ---------- ---------- ---------- ----------- ---------------  I                  9907           2 C102       C26408     .000100939           4 HEIGHT BALANCED  A                                                                                NONE  

如果要删除列已有的柱状图信息而保留列的基本统计信息，则需要重新收集bucket为1的统计信息

SQL> exec dbms_stats.gather_table_stats(user, 'TEST',  cascade=>false, method_opt=>'for columns i size 1');    PL/SQL procedure successfully completed.    SQL> select column_name,num_distinct,num_buckets,low_value,high_value,density,avg_col_len,histogram    2  from user_tab_columns where table_name='TEST';    COLUMN_NAM NUM_DISTINCT NUM_BUCKETS LOW_VALUE  HIGH_VALUE    DENSITY AVG_COL_LEN HISTOGRAM  ---------- ------------ ----------- ---------- ---------- ---------- ----------- ---------------  I                  9907           1 C102       C26408     .000100939           4 NONE  A                                                                                NONE  

这个操作明显不太合理，重新收集统计信息的代价有时候是很大的，所以Oracle11g对此做出了改进，允许只删除柱状图而保留基本统计信息，命令语法如下：

exec dbms_stats.delete_column_stats('', '  ', '  ', col_stat_type=>'HISTOGRAM');  

而要彻底删除整个列的统计信息，则需要调用delete_column_stats过程

SQL> exec dbms_stats.delete_column_stats(user, 'TEST', 'I');    PL/SQL procedure successfully completed.    SQL> select column_name,num_distinct,num_buckets,low_value,high_value,density,avg_col_len,histogram    2  from user_tab_columns where table_name='TEST';    COLUMN_NAM NUM_DISTINCT NUM_BUCKETS LOW_VALUE  HIGH_VALUE    DENSITY  AVG_COL_LEN HISTOGRAM  ---------- ------------ ----------- ---------- ---------- ----------  ----------- ---------------  I                                                                                 NONE  A                                                                                 NONE  

可以在同一个过程中收集多个列的统计信息，并且可以为不同的列指定不同的bucket个数：

SQL> exec dbms_stats.gather_table_stats(user, 'TEST',  cascade=>false, method_opt => 'for columns size 1 T for columns size 2 A');    PL/SQL procedure successfully completed.    SQL> select column_name,num_distinct,num_buckets,low_value,high_value,density,avg_col_len,histogram    2  from user_tab_columns where table_name='TEST';    COLUMN_NAM NUM_DISTINCT NUM_BUCKETS LOW_VALUE  HIGH_VALUE              DENSITY AVG_COL_LEN HISTOGRAM  ---------- ------------ ----------- ---------- -------------------- ---------- ----------- ---------------  I                  9907           1 C102       C26408               .000100939           4 NONE  A                  7376           2 41         5F75746C245F6C6E635F .000185239          18 HEIGHT BALANCED                                                 696E645F7061727473  

值得注意的是，9i的dbms_stats中，method_opt的默认值是FOR ALL COLUMNS SIZE 1，也就是收集列的基本统计信息而不收集柱状图信息，而10g的默认值则变成了FOR ALL COLUMNS SIZE AUTO，则Oracle在收集列的基本信息之外，还会根据情况收集某些列的柱状图。