Oracle11g/12c dbms_stat extended stats 实验的示例分析

蜗牛vps互联网技术资讯2021-11-292870

Oracle11g/12c dbms_stat extended stats 实验的示例分析，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。

目的：
1. 测试下 dbms_stats 的 extended stats 包的使用效果，是否可以在选择率低的情况下，解决执行计划/cardinatlity 基数行不准的问题，
并且是只验证  extended stats 包的使用效果 。 :D:

环境：
1. windows 7 64 位
2. oracle 12.2.0.1  64 位
3. 使用demo程序的 sales history , SH 用户测试。
4. 先手动安装下 sales history 范例数据

步骤：
1. sales 表先插入 50 行，
2. 分析表，获得统计信息
3. 插入大量数据，让统计信息不准
4. 使用extended status 分析包
5. 对比执行计划

步骤1-2 ：
alter session set statistics_level = all ;

set linesize 120

show parameter statistics_level ;
NAME                               TYPE                VALUE
------------------------------------ ---------------------- --------------------
statistics_level                   string                ALL
SQL>

insert into sales select * from sales_02 where rownum <= 50 ;

EXEC DBMS_STATS.GATHER_TABLE_STATS(user,'SALES');
select column_name, num_distinct, histogram from user_tab_columns where table_name = 'SALES'  ;

SQL> select column_name, num_distinct, histogram from user_tab_columns where table_name = 'SALES'  ;

COLUMN_NAM NUM_DISTINCT HISTOGRAM
---------- ------------ ------------------------------
PROD_ID             1 FREQUENCY
CUST_ID             50 NONE
TIME_ID             2 NONE
CHANNEL_ID          2 FREQUENCY
PROMO_ID             1 FREQUENCY
QUANTITY_S          1 NONE
AMOUNT_SOL          2 FREQUENCY

select num_rows from user_tables where table_name = 'SALES'  ;
SQL> select num_rows from user_tables where table_name = 'SALES'  ;

  NUM_ROWS
----------
      50

select count(*) from sales a where amount_sold >
( select avg(amount_sold)  from sales b
where b.prod_id = a.prod_id
and channel_id = 3 and promo_id = 999 )
and channel_id = 3 and promo_id = 999 ;

  COUNT(*)
----------
   309

已用时间:  00: 00: 00.02
SQL>

select * from table(dbms_xplan.display_cursor(NULL,NULL,'allstats last')) ;

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
SQL_ID  18vj1zs6jut5g, child number 0
-------------------------------------
select count(*) from sales a where amount_sold > ( select
avg(amount_sold)  from sales b where b.prod_id = a.prod_id and
channel_id = 3 and promo_id = 999 ) and channel_id = 3 and promo_id =
999

Plan hash value: 1265065521

----------------------------------------------------------------------------------------
| Id  | Operation          | Name  | Starts | E-Rows | A-Rows | A-Time | Buffers |
| 0 | SELECT STATEMENT    |    |    1 |       |    1 |00:00:00.01 |    14 |
| 1 |  SORT AGGREGATE    |    |    1 |    1 |    1 |00:00:00.01 |    14 |
|*  2 | FILTER          |    |    1 |       |    0 |00:00:00.01 |    14 |
|*  3 | TABLE ACCESS FULL | SALES |    1 |    48 |    49 |00:00:00.01 |    7 |
| 4 | SORT AGGREGATE |    |    1 |    1 |    1 |00:00:00.01 |    7 |
|*  5 |    TABLE ACCESS FULL| SALES |    1 |    48 |    49 |00:00:00.01 |    7 |
----------------------------------------------------------------------------------------


CHANNEL_ID          2 FREQUENCY
PROMO_ID             1 FREQUENCY

  NUM_ROWS
----------
   50

计算 cardinality
   1/2 * 1 * 50 = 25  ,
基数算出来是 25


步骤3：

SQL> select count(*) from sales ;

  COUNT(*)
----------
2756579

再次执行sql，并看执行计划：

select count(*) from sales a where amount_sold >
( select avg(amount_sold)  from sales b
where b.prod_id = a.prod_id
and channel_id = 3 and promo_id = 999 )
and channel_id = 3 and promo_id = 999 ;

select * from table(dbms_xplan.display_cursor(NULL,NULL,'allstats last')) ;
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------
SQL_ID  18vj1zs6jut5g, child number 0
-------------------------------------
select count(*) from sales a where amount_sold > ( select
avg(amount_sold)  from sales b where b.prod_id = a.prod_id and
channel_id = 3 and promo_id = 999 ) and channel_id = 3 and promo_id =
999

Plan hash value: 1265065521

----------------------------------------------------------------------------------------
| Id  | Operation          | Name  | Starts | E-Rows | A-Rows | A-Time | Buffers |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT    |    |    1 |       |    1 |00:01:53.45 | 2336K|
| 1 |  SORT AGGREGATE    |    |    1 |    1 |    1 |00:01:53.45 | 2336K|
|*  2 | FILTER          |    |    1 |       | 709K|00:01:53.33 | 2336K|
|*  3 | TABLE ACCESS FULL | SALES |    1 |    48 | 1554K|00:00:00.98 | 13431 |
| 4 | SORT AGGREGATE |    | 173 |    1 | 173 |00:01:51.30 | 2323K|
|*  5 |    TABLE ACCESS FULL| SALES | 173 |    48 | 2131K|00:01:50.93 | 2323K|
----------------------------------------------------------------------------------------

48 VS  2131000  = 50000 倍

SQL> select num_rows from user_tables where table_name = 'SALES'  ;

  NUM_ROWS
----------
      50

使用 extended 包
select dbms_stats.create_extended_stats(ownname => 'SH' , tabname => 'SALES' , extension => '(CHANNEL_ID,PROMO_ID)' ) from dual ;

DBMS_STATS.CREATE_EXTENDED_STATS(OWNNAME=>'SH',TABNAME=>'SALES',EXTENSION=>'(CHANNEL_ID,PROMO_ID)')
------------------------------------------------------------------------------------------------------------------------
SYS_STU7$MLVU9QOBUF89709XS1VC9

已用时间:  00: 00: 01.65
SQL>
SQL>

exec dbms_stats.gather_table_stats(null,'SALES', method_opt => 'for columns SYS_STU7$MLVU9QOBUF89709XS1VC9 size 2' );

select count(*) from sales a where amount_sold >
( select avg(amount_sold)  from sales b
where b.prod_id = a.prod_id
and channel_id = 3 and promo_id = 999 )
and channel_id = 3 and promo_id = 999 ;

alter system flush shared_pool;
select * from table(dbms_xplan.display_cursor(NULL,NULL,'allstats last')) ;

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------
SQL_ID  18vj1zs6jut5g, child number 0
-------------------------------------
select count(*) from sales a where amount_sold > ( select
avg(amount_sold)  from sales b where b.prod_id = a.prod_id and
channel_id = 3 and promo_id = 999 ) and channel_id = 3 and promo_id =
999

Plan hash value: 4009253081

----------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |  OMem |  1Mem | Used-Mem |
| 0 | SELECT STATEMENT    |       |    1 |       |    1 |00:00:01.79 | 26898 |    | |          |
| 1 |  SORT AGGREGATE    |       |    1 |    1 |    1 |00:00:01.79 | 26898 |    | |          |
|*  2 | HASH JOIN          |       |    1 |  94910 | 709K|00:00:01.79 | 26898 |  1316K|  1316K| 1666K (0)|
| 3 | VIEW             | VW_SQ_1 |    1 |    72 |    72 |00:00:00.92 | 13431 |    | |          |
| 4 |    HASH GROUP BY    |       |    1 |    72 |    72 |00:00:00.92 | 13431 |  1106K|  1106K| 2480K (0)|
|*  5 |    TABLE ACCESS FULL| SALES |    1 | 1554K| 1554K|00:00:00.48 | 13431 |    | |          |
|*  6 | TABLE ACCESS FULL  | SALES |    1 | 1554K| 1554K|00:00:00.49 | 13431 |    | |          |
----------------------------------------------------------------------------------------------------------------------

SQL> select num_rows from user_tables where table_name = 'SALES'  ;

  NUM_ROWS
----------
2756579

EXEC DBMS_STATS.GATHER_TABLE_STATS(user,'SALES');

SQL> select column_name, num_distinct, histogram from user_tab_columns where table_name = 'SALES'  ;

COLUMN_NAM NUM_DISTINCT HISTOGRAM
---------- ------------ ------------------------------
PROD_ID             72 FREQUENCY
CUST_ID          7059 NONE
TIME_ID          1460 NONE
CHANNEL_ID          4 FREQUENCY
PROMO_ID             4 FREQUENCY
QUANTITY_S          1 NONE
AMOUNT_SOL       3586 HYBRID

已选择 7 行。
CHANNEL_ID  PROMO_ID 各选择一个值，就是：

1/4 * 1/4 * 2756579 =  172286

基数是 172286
estimated rows : 1554000

去掉统计信息

这些都无效，
exec DBMS_STATS.DROP_EXTENDED_STATS(null,'SALES','(CHANNEL_ID,PROMO_ID)');
exec DBMS_STATS.DELETE_TABLE_STATS(ownname => 'SH',  tabname => 'SALES');
select ADDRESS, HASH_VALUE from V$SQLAREA where SQL_ID = '18vj1zs6jut5g';
ADDRESS       HASH_VALUE
---------------- ----------
000007FF0465AB40  220030127
exec SYS.DBMS_SHARED_POOL.PURGE ('000007FF0465AB40,220030127', 'C');

select count(*) from sales a where amount_sold >
( select avg(amount_sold)  from sales b
where b.prod_id = a.prod_id
and channel_id = 3 and promo_id = 999 )
and channel_id = 3 and promo_id = 999 ;

SQL> select column_name, num_distinct, histogram from user_tab_columns where table_name = 'SALES'  ;

COLUMN_NAM NUM_DISTINCT HISTOGRAM
---------- ------------ ------------------------------
PROD_ID                NONE
CUST_ID                NONE
TIME_ID                NONE
CHANNEL_ID             NONE
PROMO_ID             NONE
QUANTITY_S             NONE
AMOUNT_SOL             NONE

已选择 7 行。

SQL>  select num_rows from user_tables where table_name = 'SALES'  ;

  NUM_ROWS
----------

select * from table(dbms_xplan.display_cursor(NULL,NULL,'allstats last')) ;

Plan hash value: 4009253081

----------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |  OMem |  1Mem | Used-Mem |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT    |       |    1 |       |    1 |00:00:01.67 | 26898 |    | |          |
| 1 |  SORT AGGREGATE    |       |    1 |    1 |    1 |00:00:01.67 | 26898 |    | |          |
|*  2 | HASH JOIN          |       |    1 |  95637 | 709K|00:00:01.67 | 26898 |  1316K|  1316K| 1583K (0)|
| 3 | VIEW             | VW_SQ_1 |    1 |    72 |    72 |00:00:00.87 | 13431 |    | |          |
| 4 |    HASH GROUP BY    |       |    1 |    72 |    72 |00:00:00.87 | 13431 |  1106K|  1106K| 2480K (0)|
|*  5 |    TABLE ACCESS FULL| SALES |    1 | 1566K| 1554K|00:00:00.45 | 13431 |    | |          |
|*  6 | TABLE ACCESS FULL  | SALES |    1 | 1566K| 1554K|00:00:00.45 | 13431 |    | |          |
----------------------------------------------------------------------------------------------------------------------

最后还是truncate table ，重新建立数据

truncate table sales ;

insert into sales select * from sales_02 where rownum <= 50 ;

EXEC DBMS_STATS.GATHER_TABLE_STATS(user,'SALES');

SQL> select column_name, num_distinct, histogram from user_tab_columns where table_name = 'SALES'  ;

COLUMN_NAM NUM_DISTINCT HISTOGRAM
---------- ------------ ------------------------------
PROD_ID             1 FREQUENCY
CUST_ID             50 NONE
TIME_ID             2 NONE
CHANNEL_ID          2 FREQUENCY
PROMO_ID             1 FREQUENCY
QUANTITY_S          1 NONE
AMOUNT_SOL          2 FREQUENCY

已选择 7 行。

SQL> select num_rows from user_tables where table_name = 'SALES'  ;

  NUM_ROWS
----------
      50

select count(*) from sales a where amount_sold >
( select avg(amount_sold)  from sales b
where b.prod_id = a.prod_id
and channel_id = 3 and promo_id = 999 )
and channel_id = 3 and promo_id = 999 ;
COUNT(*)
---------
709087

select * from table(dbms_xplan.display_cursor(NULL,NULL,'allstats last')) ;
Plan hash value: 1265065521

----------------------------------------------------------------------------------------
| Id  | Operation          | Name  | Starts | E-Rows | A-Rows | A-Time | Buffers |

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT    |    |    1 |       |    1 |00:01:32.27 | 2339K|
| 1 |  SORT AGGREGATE    |    |    1 |    1 |    1 |00:01:32.27 | 2339K|
|*  2 | FILTER          |    |    1 |       | 709K|00:01:32.13 | 2339K|
|*  3 | TABLE ACCESS FULL | SALES |    1 |    1 | 1554K|00:00:00.67 | 13571 |
| 4 | SORT AGGREGATE |    | 173 |    1 | 173 |00:01:30.73 | 2326K|
|*  5 |    TABLE ACCESS FULL| SALES | 173 |    1 | 2131K|00:01:30.44 | 2326K|
----------------------------------------------------------------------------------------

1 VS 2131k 差无数倍

1 - SEL$1
3 - SEL$1 / A@SEL$1
4 - SEL$2
5 - SEL$2 / B@SEL$2

使用  hint

select /*+UNNEST(@"SEL$2")*/ count(*) from sales a where amount_sold >
( select avg(amount_sold)  from sales b
where b.prod_id = a.prod_id
and channel_id = 3 and promo_id = 999 )
and channel_id = 3 and promo_id = 999 ;
select * from table(dbms_xplan.display_cursor(NULL,NULL,'allstats last')) ;

Plan hash value: 4009253081

----------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows | A-Time |

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT    |       |    1 |       |    1 |00:00:01.64 |
| 1 |  SORT AGGREGATE    |       |    1 |    1 |    1 |00:00:01.64 |
|*  2 | HASH JOIN          |       |    1 |    2 | 709K|00:00:01.64 |
| 3 | VIEW             | VW_SQ_1 |    1 |    1 |    72 |00:00:00.85 |
| 4 |    HASH GROUP BY    |       |    1 |    1 |    72 |00:00:00.85 |
|*  5 |    TABLE ACCESS FULL| SALES |    1 |    48 | 1554K|00:00:00.44 |
|*  6 | TABLE ACCESS FULL  | SALES |    1 |    48 | 1554K|00:00:00.44 |
----------------------------------------------------------------------------------