Question

我的要求：确定DEPT_NUM的前10个帐户，按帐号按升序排序。

查询：

SELECT * FROM
(
  select acctnum,dept_num,row_number() OVER (PARTITION BY DEPT_NUM ORDER BY ACCTNUM) as row_identifier
   FROM MYTABLE
)
WHERE row_identifier between 1 and 10;

跟踪：

    7532 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1480074522

--------------------------------------------------------------------------------------------
| Id  | Operation                | Name    | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |         |   577K|    15M|       |  3855   (1)| 00:00:47 |
|*  1 |  VIEW                    |         |   577K|    15M|       |  3855   (1)| 00:00:47 |
|*  2 |   WINDOW SORT PUSHED RANK|         |   577K|  7890K|    13M|  3855   (1)| 00:00:47 |
|   3 |    INDEX FAST FULL SCAN  | IMTAB05 |   577K|  7890K|       |   987   (1)| 00:00:12 |
--------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("ROW_IDENTIFIER">=1 AND "ROW_IDENTIFIER"<=5)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY "DEPT_NUM" ORDER BY "ACCTNUM")<=5)


Statistics
----------------------------------------------------------
          0  recursive calls
          2  db block gets
       4298  consistent gets
          0  physical reads
          0  redo size
     144367  bytes sent via SQL*Net to client
        486  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
       7532  rows processed

索引：

index scan列INDEX STORAGE上的DEPT_NUM。

强制Full Table扫描费用从3855到11092

表中的总行数为632667;

以上都是测试区域结果。生产实际上有两倍的数量。

我的数据库是Exadata，Quarter RAC。运行Oracle 11g R2。数据库的功能足以立即执行，但DBA不愿意使用13M的tempSpc。据业务报道，此报告的频率为每小时4次。主要的是，这个表有很多实时插入/更新

我们可以像锡一样即兴发挥这个过程 1）增加会话的PGA？（不确定，如果真的可能吗？）
2）另外一个索引会有帮助吗？

只是想要一些不同的眼睛看这个，因为我们的团队只关注DBA参数。

感谢您提出任何建议！

Answer 1

分析函数性能可能取决于索引列顺序。将索引从(ACCTNUM,DEPT_NUM)更改为(DEPT_NUM,ACCTNUM)可能会降低成本并消除对临时表空间的需求。

partition by COL_2 order by COL_1 => INDEX FAST FULL SCAN|WINDOW SORT PUSHED RANK
partition by COL_1 order by COL_2 => INDEX FULL SCAN|WINDOW NOSORT

INDEX FAST FULL SCAN使用更快的多块IO，但它还需要为排序区域排序数据和可能的临时表空间。

INDEX FULL SCAN使用较慢的单块IO，但它按顺序返回数据并避免排序。

示例架构和数据

--drop table mytable;
create table mytable(dept_num number not null, acctnum number not null
    ,a number, b number, c number, d number, e number);
insert into mytable
select 1 dept_num, 1 acctnum, 0,0,0,0,0 from dual union all
select 1 dept_num, 2 acctnum, 0,0,0,0,0 from dual union all
select 1 dept_num, 3 acctnum, 0,0,0,0,0 from dual union all
select 2 dept_num, 1 acctnum, 0,0,0,0,0 from dual union all
select 2 dept_num, 2 acctnum, 0,0,0,0,0 from dual union all
select 3 dept_num, 1 acctnum, 0,0,0,0,0 from dual;
--Create 600K similar rows.
insert into mytable
  select dept_num + rownumber*3, acctnum, a,b,c,d,e
  from mytable
  cross join (select level rownumber from dual connect by level <= 100000);
begin
    dbms_stats.gather_table_stats(user, 'mytable');
end;
/

（ACCTNUM，DEPT_NUM）= WINDOW SORT PUSHED RANK

create index mytable_idx on mytable(acctnum, dept_num);

explain plan for
select dept_num, acctnum from
(
    select dept_num, acctnum
        ,row_number() over (partition by dept_num order by acctnum) as row_identifier
    from mytable
)
where row_identifier between 1 and 10;

select * from table(dbms_xplan.display);

Plan hash value: 952182109

------------------------------------------------------------------------------------------------
| Id  | Operation                | Name        | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |             |   600K|    22M|       |  1625   (3)| 00:00:23 |
|*  1 |  VIEW                    |             |   600K|    22M|       |  1625   (3)| 00:00:23 |
|*  2 |   WINDOW SORT PUSHED RANK|             |   600K|  4687K|  9424K|  1625   (3)| 00:00:23 |
|   3 |    INDEX FAST FULL SCAN  | MYTABLE_IDX |   600K|  4687K|       |   239   (3)| 00:00:04 |
------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("ROW_IDENTIFIER">=1 AND "ROW_IDENTIFIER"<=10)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY "DEPT_NUM" ORDER BY "ACCTNUM")<=10)

（DEPT_NUM，ACCTNUM）=窗口不适用

drop index mytable_idx;
create index mytable_idx on mytable(dept_num, acctnum);

explain plan for
select dept_num, acctnum from
(
    select dept_num, acctnum
        ,row_number() over (partition by dept_num order by acctnum) as row_identifier
    from mytable
)
where row_identifier between 1 and 10;

select * from table(dbms_xplan.display);

Plan hash value: 1773829932

---------------------------------------------------------------------------------
| Id  | Operation         | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |             |   600K|    22M|   792   (2)| 00:00:12 |
|*  1 |  VIEW             |             |   600K|    22M|   792   (2)| 00:00:12 |
|*  2 |   WINDOW NOSORT   |             |   600K|  4687K|   792   (2)| 00:00:12 |
|   3 |    INDEX FULL SCAN| MYTABLE_IDX |   600K|  4687K|   792   (2)| 00:00:12 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("ROW_IDENTIFIER">=1 AND "ROW_IDENTIFIER"<=10)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY "DEPT_NUM" ORDER BY 
              "ACCTNUM")<=10)

窗口功能很昂贵，我们可以克服吗？

1 个答案: