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。
