[20180122]列统计与直方图.txt

--//昨天看了https://jonathanlewis.wordpress.com/2018/01/18/column-stats/,提到分析method的各种写法,自己重复验证看看:

1.环境:
SCOTT@book> @ ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

create table t1 as select object_type o1, object_type o2, object_type o3, object_id, object_name from all_objects where rownum <= 50000;

SCOTT@book> select  column_name, num_distinct, histogram, num_buckets, last_analyzed from  user_tab_cols where table_name = 'T1' order by column_id;
COLUMN_NAME          NUM_DISTINCT HISTOGRAM       NUM_BUCKETS LAST_ANALYZED
-------------------- ------------ --------------- ----------- -------------------
O1                                NONE
O2                                NONE
O3                                NONE
OBJECT_ID                         NONE
OBJECT_NAME                       NONE

--//建立后没有分析.

2.测试:

exec dbms_stats.gather_table_stats(user,'t1',method_opt=>'for all columns size 1 for columns o1 o2 o3 size 15');

SCOTT@book> select  column_name, num_distinct, histogram, num_buckets, last_analyzed,histogram from  user_tab_cols where table_name = 'T1' order by column_id;
COLUMN_NAME          NUM_DISTINCT HISTOGRAM       NUM_BUCKETS LAST_ANALYZED       HISTOGRAM
-------------------- ------------ --------------- ----------- ------------------- ---------------
O1                             33 NONE                      1 2018-01-22 08:52:44 NONE
O2                             33 NONE                      1 2018-01-22 08:52:44 NONE
O3                             33 HEIGHT BALANCED          15 2018-01-22 08:52:44 HEIGHT BALANCED
OBJECT_ID                   50000 NONE                      1 2018-01-22 08:52:44 NONE
OBJECT_NAME                 28576 NONE                      1 2018-01-22 08:52:44 NONE
--//注意看O1,O2字段没有建立直方图.

SCOTT@book> exec dbms_stats.gather_table_stats(user,'t1',method_opt=>'for all columns size 1 for columns size 15 o1 o2 o3');
PL/SQL procedure successfully completed.

SCOTT@book> select  column_name, num_distinct, histogram, num_buckets, last_analyzed,histogram from  user_tab_cols where table_name = 'T1' order by column_id;
COLUMN_NAME          NUM_DISTINCT HISTOGRAM       NUM_BUCKETS LAST_ANALYZED       HISTOGRAM
-------------------- ------------ --------------- ----------- ------------------- ---------------
O1                             33 HEIGHT BALANCED          15 2018-01-22 08:57:37 HEIGHT BALANCED
O2                             33 HEIGHT BALANCED          15 2018-01-22 08:57:37 HEIGHT BALANCED
O3                             33 HEIGHT BALANCED          15 2018-01-22 08:57:37 HEIGHT BALANCED
OBJECT_ID                   50000 NONE                      1 2018-01-22 08:57:37 NONE
OBJECT_NAME                 28576 NONE                      1 2018-01-22 08:57:37 NONE
--//这样O1,O2,O3字段建立直方图.

--//我个人喜欢的写法是:
SCOTT@book> exec dbms_stats.gather_table_stats(user,'t1',method_opt=>'for all columns size 1 for columns o1 size 15 o2 size 16 o3 size 17');
PL/SQL procedure successfully completed.

SCOTT@book> select  column_name, num_distinct, histogram, num_buckets, last_analyzed,histogram from  user_tab_cols where table_name = 'T1' order by column_id;
COLUMN_NAME          NUM_DISTINCT HISTOGRAM       NUM_BUCKETS LAST_ANALYZED       HISTOGRAM
-------------------- ------------ --------------- ----------- ------------------- ---------------
O1                             33 HEIGHT BALANCED          15 2018-01-22 08:58:46 HEIGHT BALANCED
O2                             33 HEIGHT BALANCED          16 2018-01-22 08:58:46 HEIGHT BALANCED
O3                             33 HEIGHT BALANCED          17 2018-01-22 08:58:46 HEIGHT BALANCED
OBJECT_ID                   50000 NONE                      1 2018-01-22 08:58:46 NONE
OBJECT_NAME                 28576 NONE                      1 2018-01-22 08:58:46 NONE
--//这样O1,O2,O3字段建立直方图.作者提倡的写法: "for columns {size clause} {column list}".这样反过来我个人不习惯.^_^.

--//作者还给出一些例子,大家执行测试:
for columns o1 size 12, o2 size 13, o3 size 14
for columns o1 size 15 o2 size 16 o3 size 17
for columns size 18 o1 size 19 o2 size 20 o3
for columns size 21 o1 o2 size 22 o3
for columns o1 size 12, o2 size 12, o3 size 13, object_id size 13 object_name size 14
for columns size 22 o1 o2 for columns size 23 o3 object_id for columns size 24  object_name

--//总之注意这样写法'for all columns size 1 for columns size 15 o1 o2 o3'可能不满足需要.在工作中要引起足够的注意.