Oracle统计信息的收集、管理与清除（作者EYGLE)

以下测试环境为Oracle 10g 10.2.0.4版本，测试对Oracle的统计信息的收集与管理。

首先依据dba_objects创建一张测试表：

SQL> create table eygle as select * from dba_objects;
 
Table created

对该表进行统计信息收集，这里未指定method_opt，则Oracle将采用 FOR ALL COLUMNS SIZE AUTO 选项，这一选项在Oracle 9i中不收集列的柱状图信息，在Oracle 10g中则会根据数据库的选项选择是否收集柱状图，缺省的会为列收集基本信息。
在Oracle 10g中，缺省的初始化参数 _column_tracking_level 设置为1，Oracle会动态收集列的使用情况，如果某些倾斜列被频繁使用，则Oracle会在Auto模式下，自动为该列收集列级柱状图。

看以下测试，执行缺省的表统计信息收集：
 

SQL> exec dbms_stats.gather_table_stats(user,'EYGLE');
 
PL/SQL procedure successfully completed

此时数据库为表上的所有字段收集了缺省的统计信息，每个列两个Bucket：

SQL> col column_name for a30
SQL> col owner for a10
SQL> col table_name for a10
SQL> col ENDPOINT_ACTUAL_VALUE for a10
SQL> select * from dba_tab_histograms where table_name='EYGLE' and owner='EYGLE' order by 4;
 
OWNER      TABLE_NAME COLUMN_NAME                    ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- ------------------------------ --------------- -------------- ----------
EYGLE      EYGLE      OWNER                                        0 3.492486153566
EYGLE      EYGLE      OBJECT_NAME                                  0 3.388431933833
EYGLE      EYGLE      SECONDARY                                    0 4.049991549657
EYGLE      EYGLE      GENERATED                                    0 4.049991549657
EYGLE      EYGLE      TEMPORARY                                    0 4.049991549657
EYGLE      EYGLE      STATUS                                       0 4.478619304731
EYGLE      EYGLE      TIMESTAMP                                    0 2.555831764971
EYGLE      EYGLE      LAST_DDL_TIME                                0 2455466.759085
EYGLE      EYGLE      CREATED                                      0 2455466.759085
EYGLE      EYGLE      OBJECT_TYPE                                  0 3.494321128346
EYGLE      EYGLE      DATA_OBJECT_ID                               0              2
EYGLE      EYGLE      OBJECT_ID                                    0              2
EYGLE      EYGLE      SUBOBJECT_NAME                               0 4.163573043437
EYGLE      EYGLE      OWNER                                        1 4.532981758140
EYGLE      EYGLE      GENERATED                                    1 4.621144204096
EYGLE      EYGLE      TEMPORARY                                    1 4.621144204096
EYGLE      EYGLE      STATUS                                       1 4.478619304731
EYGLE      EYGLE      TIMESTAMP                                    1 2.605922956775
EYGLE      EYGLE      LAST_DDL_TIME                                1 2455492.879583
EYGLE      EYGLE      SECONDARY                                    1 4.049991549657
EYGLE      EYGLE      OBJECT_TYPE                                  1 4.532166435311
EYGLE      EYGLE      DATA_OBJECT_ID                               1          12918
EYGLE      EYGLE      OBJECT_ID                                    1          12918
EYGLE      EYGLE      SUBOBJECT_NAME                               1 4.533986999644
EYGLE      EYGLE      OBJECT_NAME                                  1 4.956504674196
EYGLE      EYGLE      CREATED                                      1 2455492.879583
 
26 rows selected
 
SQL> select * from dba_tab_histograms where table_name='EYGLE' and owner='EYGLE' order by 3;
 
OWNER      TABLE_NAME COLUMN_NAME                    ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- ------------------------------ --------------- -------------- ----------
EYGLE      EYGLE      CREATED                                      0 2455466.759085
EYGLE      EYGLE      CREATED                                      1 2455492.879583
EYGLE      EYGLE      DATA_OBJECT_ID                               0              2
EYGLE      EYGLE      DATA_OBJECT_ID                               1          12918
EYGLE      EYGLE      GENERATED                                    0 4.049991549657
EYGLE      EYGLE      GENERATED                                    1 4.621144204096
EYGLE      EYGLE      LAST_DDL_TIME                                0 2455466.759085
EYGLE      EYGLE      LAST_DDL_TIME                                1 2455492.879583
EYGLE      EYGLE      OBJECT_ID                                    1          12918
EYGLE      EYGLE      OBJECT_ID                                    0              2
EYGLE      EYGLE      OBJECT_NAME                                  0 3.388431933833
EYGLE      EYGLE      OBJECT_NAME                                  1 4.956504674196
EYGLE      EYGLE      OBJECT_TYPE                                  1 4.532166435311
EYGLE      EYGLE      OBJECT_TYPE                                  0 3.494321128346
EYGLE      EYGLE      OWNER                                        0 3.492486153566
EYGLE      EYGLE      OWNER                                        1 4.532981758140
EYGLE      EYGLE      SECONDARY                                    1 4.049991549657
EYGLE      EYGLE      SECONDARY                                    0 4.049991549657
EYGLE      EYGLE      STATUS                                       0 4.478619304731
EYGLE      EYGLE      STATUS                                       1 4.478619304731
 
OWNER      TABLE_NAME COLUMN_NAME                    ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- ------------------------------ --------------- -------------- ----------
EYGLE      EYGLE      SUBOBJECT_NAME                               1 4.533986999644
EYGLE      EYGLE      SUBOBJECT_NAME                               0 4.163573043437
EYGLE      EYGLE      TEMPORARY                                    0 4.049991549657
EYGLE      EYGLE      TEMPORARY                                    1 4.621144204096
EYGLE      EYGLE      TIMESTAMP                                    0 2.555831764971
EYGLE      EYGLE      TIMESTAMP                                    1 2.605922956775
 
26 rows selected

同时，列的低值、高值等信息会被收集记录在dba_tab_col_statistics中：

SQL> select table_name,column_name,num_distinct,low_value,high_value,DENSITY from dba_tab_col_statistics
  2  where table_name='EYGLE';
 
TABLE_NAME COLUMN_NAME          NUM_DISTINCT LOW_VALUE          HIGH_VALUE                    DENSITY
---------- -------------------- ------------ ------------------ -------------------------- ----------
EYGLE      OWNER                          11 43434943           574D535953                 0.09090909
EYGLE      OBJECT_NAME                  9096 4142425F464A31     5F75746C245F6C6E635F696E64 0.00010993
EYGLE      SUBOBJECT_NAME                 79 5030               575248245F5741495453545F35 0.01265822
EYGLE      OBJECT_ID                   11676 C103               C3021E13                   8.56457690
EYGLE      DATA_OBJECT_ID               2869 C103               C3021E13                   0.00034855
EYGLE      OBJECT_TYPE                    35 434C5553544552     57494E444F572047524F5550   0.02857142
EYGLE      CREATED                       494 786E091A130E06     786E0A16160725             0.00202429
EYGLE      LAST_DDL_TIME                 502 786E091A130E06     786E0A16160725             0.00199203
EYGLE      TIMESTAMP                     515 313939372D30342D31 323031302D31302D32323A3231 0.00194174
EYGLE      STATUS                          1 56414C4944         56414C4944                          1
EYGLE      TEMPORARY                       2 4E                 59                                0.5
EYGLE      GENERATED                       2 4E                 59                                0.5
EYGLE      SECONDARY                       1 4E                 4E                                  1

这些基本信息在执行计划生成时会被参考，不能通过for all columns size 1的收集方式删除：

SQL> exec dbms_stats.gather_table_stats(user,'EYGLE',method_opt => 'for all columns size 1');
 
PL/SQL procedure successfully completed

SQL> select * from dba_tab_histograms where table_name='EYGLE' and owner='EYGLE' order by 3;
 
OWNER      TABLE_NAME COLUMN_NAME          ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- -------------------- --------------- -------------- ----------
EYGLE      EYGLE      CREATED                            0 2455466.759085
EYGLE      EYGLE      CREATED                            1 2455492.879583
EYGLE      EYGLE      DATA_OBJECT_ID                     0              2
EYGLE      EYGLE      DATA_OBJECT_ID                     1          12918
EYGLE      EYGLE      GENERATED                          0 4.049991549657
EYGLE      EYGLE      GENERATED                          1 4.621144204096
EYGLE      EYGLE      LAST_DDL_TIME                      0 2455466.759085
EYGLE      EYGLE      LAST_DDL_TIME                      1 2455492.879583
EYGLE      EYGLE      OBJECT_ID                          1          12918
EYGLE      EYGLE      OBJECT_ID                          0              2
EYGLE      EYGLE      OBJECT_NAME                        0 3.388431933833
EYGLE      EYGLE      OBJECT_NAME                        1 4.956504674196
EYGLE      EYGLE      OBJECT_TYPE                        1 4.532166435311
EYGLE      EYGLE      OBJECT_TYPE                        0 3.494321128346
EYGLE      EYGLE      OWNER                              0 3.492486153566
EYGLE      EYGLE      OWNER                              1 4.532981758140
EYGLE      EYGLE      SECONDARY                          1 4.049991549657
EYGLE      EYGLE      SECONDARY                          0 4.049991549657
EYGLE      EYGLE      STATUS                             0 4.478619304731
EYGLE      EYGLE      STATUS                             1 4.478619304731
 
OWNER      TABLE_NAME COLUMN_NAME          ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- -------------------- --------------- -------------- ----------
EYGLE      EYGLE      SUBOBJECT_NAME                     1 4.533986999644
EYGLE      EYGLE      SUBOBJECT_NAME                     0 4.163573043437
EYGLE      EYGLE      TEMPORARY                          0 4.049991549657
EYGLE      EYGLE      TEMPORARY                          1 4.621144204096
EYGLE      EYGLE      TIMESTAMP                          0 2.555831764971
EYGLE      EYGLE      TIMESTAMP                          1 2.605922956775

类似如下删除单列统计信息的语句也对基本统计信息无效：

SQL> exec dbms_stats.gather_table_stats(user,'EYGLE',method_opt=>'for columns owner size 1');
 
PL/SQL procedure successfully completed

但是使用delete_column_stats可以彻底删除列的柱状图信息：

SQL> exec dbms_stats.delete_column_stats(user,'EYGLE','OWNER');
 
PL/SQL procedure successfully completed

SQL> select * from dba_tab_histograms where table_name='EYGLE' and owner='EYGLE' order by 3;
 
OWNER      TABLE_NAME COLUMN_NAME          ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- -------------------- --------------- -------------- ----------
EYGLE      EYGLE      CREATED                            1 2455492.879583
EYGLE      EYGLE      CREATED                            0 2455466.759085
EYGLE      EYGLE      DATA_OBJECT_ID                     0              2
EYGLE      EYGLE      DATA_OBJECT_ID                     1          12918
EYGLE      EYGLE      GENERATED                          0 4.049991549657
EYGLE      EYGLE      GENERATED                          1 4.621144204096
EYGLE      EYGLE      LAST_DDL_TIME                      0 2455466.759085
EYGLE      EYGLE      LAST_DDL_TIME                      1 2455492.879583
EYGLE      EYGLE      OBJECT_ID                          0              2
EYGLE      EYGLE      OBJECT_ID                          1          12918
EYGLE      EYGLE      OBJECT_NAME                        0 3.388431933833
EYGLE      EYGLE      OBJECT_NAME                        1 4.956504674196
EYGLE      EYGLE      OBJECT_TYPE                        1 4.532166435311
EYGLE      EYGLE      OBJECT_TYPE                        0 3.494321128346
EYGLE      EYGLE      SECONDARY                          1 4.049991549657
EYGLE      EYGLE      SECONDARY                          0 4.049991549657
EYGLE      EYGLE      STATUS                             0 4.478619304731
EYGLE      EYGLE      STATUS                             1 4.478619304731
EYGLE      EYGLE      SUBOBJECT_NAME                     0 4.163573043437
EYGLE      EYGLE      SUBOBJECT_NAME                     1 4.533986999644
 
OWNER      TABLE_NAME COLUMN_NAME          ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- -------------------- --------------- -------------- ----------
EYGLE      EYGLE      TEMPORARY                          0 4.049991549657
EYGLE      EYGLE      TEMPORARY                          1 4.621144204096
EYGLE      EYGLE      TIMESTAMP                          0 2.555831764971
EYGLE      EYGLE      TIMESTAMP                          1 2.605922956775
 
24 rows selected

但是通常Oracle不建议删除列上的基本统计信息，因为这些信息在进行执行计划选择时可能极为有用，比如判断某些谓词的取值是否越界等。

看以下操作，先完整收集13列26行统计信息：

SQL> exec dbms_stats.gather_table_stats(user,'EYGLE');
 
PL/SQL procedure successfully completed

然后在表上执行基于OWNER的查询：

SQL> select count(*) from eygle where owner='SYS';
 
  COUNT(*)
----------
      6729
 
SQL> select count(*) from eygle where owner='EYGLE';
 
  COUNT(*)
----------
       240
 
SQL> select count(*) from eygle where owner='SYSTEM';
 
  COUNT(*)
----------
       449

再来进行缺省条件的统计信息收集：

SQL> exec dbms_stats.gather_table_stats(user,'EYGLE');
 
PL/SQL procedure successfully completed

此时你可能会注意到，OWNER列的柱状图已经被收集：

SQL> select * from dba_tab_histograms where table_name='EYGLE' and owner='EYGLE' order by 3;
 
OWNER      TABLE_NAME COLUMN_NAME          ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ---------- -------------------- --------------- -------------- ----------
EYGLE      EYGLE      CREATED                            1 2455492.879583
EYGLE      EYGLE      CREATED                            0 2455466.759085
EYGLE      EYGLE      DATA_OBJECT_ID                     1          12918
EYGLE      EYGLE      DATA_OBJECT_ID                     0              2
EYGLE      EYGLE      GENERATED                          1 4.621144204096
EYGLE      EYGLE      GENERATED                          0 4.049991549657
EYGLE      EYGLE      LAST_DDL_TIME                      0 2455466.759085
EYGLE      EYGLE      LAST_DDL_TIME                      1 2455492.879583
EYGLE      EYGLE      OBJECT_ID                          1          12918
EYGLE      EYGLE      OBJECT_ID                          0              2
EYGLE      EYGLE      OBJECT_NAME                        1 4.956504674196
EYGLE      EYGLE      OBJECT_NAME                        0 3.388431933833
EYGLE      EYGLE      OBJECT_TYPE                        1 4.532166435311
EYGLE      EYGLE      OBJECT_TYPE                        0 3.494321128346
EYGLE      EYGLE      OWNER                            312 4.119221354213
EYGLE      EYGLE      OWNER                           2842 4.171130061672
EYGLE      EYGLE      OWNER                          11679 4.532981758140
EYGLE      EYGLE      OWNER                           9571 4.327723496506
EYGLE      EYGLE      OWNER                          11361 4.327723757311
EYGLE      EYGLE      OWNER                          11364 4.378425024777
EYGLE      EYGLE      OWNER                          10912 4.327723735598
EYGLE      EYGLE      OWNER                              9 3.492486153566
EYGLE      EYGLE      OWNER                             55 3.544214255849
EYGLE      EYGLE      OWNER                            295 3.600792664974
EYGLE      EYGLE      OWNER                            303 4.118597800700
EYGLE      EYGLE      SECONDARY                          0 4.049991549657
EYGLE      EYGLE      SECONDARY                          1 4.049991549657
EYGLE      EYGLE      STATUS                             0 4.478619304731
EYGLE      EYGLE      STATUS                             1 4.478619304731
EYGLE      EYGLE      SUBOBJECT_NAME                     0 4.163573043437
EYGLE      EYGLE      SUBOBJECT_NAME                     1 4.533986999644
EYGLE      EYGLE      TEMPORARY                          1 4.621144204096
EYGLE      EYGLE      TEMPORARY                          0 4.049991549657
EYGLE      EYGLE      TIMESTAMP                          1 2.605922956775
EYGLE      EYGLE      TIMESTAMP                          0 2.555831764971
 
35 rows selected

这种现象就来源于 _column_tracking_level 的特性监控，如果不希望发生这样的信息收集，则可以调整这个隐含的参数。

此时再使用delete_column_stats就可以删除这些字段的统计信息：

SQL> exec dbms_stats.delete_column_stats(user,'EYGLE','OWNER');
 
PL/SQL procedure successfully completed

也可以针对这个列使用如下命令清除这个字段的柱状图但是保留基本统计信息：

SQL> exec dbms_stats.gather_table_stats(user,'EYGLE',method_opt=>'for columns owner size 1');
 
PL/SQL procedure successfully completed

通常推荐使用'for columns owner size 1' 而不是delete_column_stats去清除列的统计信息，完全删除列的基本统计信息在某些Bug的作用下，可能会导致优化器计算的异常。

在统计信息收集时，必须注意到这些选项和后台动作，否则就可能出现和面对很多莫名其妙的问题。