Question

我总是想知道在某个外键的历史记录表中找到最后一个条目的正确方法是什么？

示例：

我们假设我们有一个这样的表：

tHistory（ID | FK_User | State | Timestamp）

在Oracle中读取每个FK_User ID的lates条目的正确方法是什么？我总是使用一个子选择，但它看起来不对我...还有其他方法吗？

Answer 1

另一种方法是使用相关的子查询来确定最后的日期......

SELECT
  *
FROM
  tHistory
WHERE
  timestamp = (
    SELECT
      MAX(timestamp)
    FROM
      tHistory latest
    WHERE
      FK_User = tHistory.FK_User
  )

Answer 2

SELECT  *
FROM    (
        SELECT  h.*,
                ROW_NUMBER() OVER 
                (PARTITION BY h.FK_User ORDER BY timestamp DESC) AS rn
        FROM    tHistory h
        )
WHERE   rn = 1

Answer 3

好问题。

在版本8i之前，您需要使用子查询，您需要两次访问thistory表。引入分析函数时，您可以使用内联视图中的ROW_NUMBER分析函数跳过第二个表访问，就像Quassnoi向您展示的那样。

但是，从版本9开始，Oracle中有一种更好，更高效的方式。只需按FK_USER分组并使用聚合函数FIRST或LAST。

第一：http://download.oracle.com/docs/cd/B10501_01/server.920/a96540/functions45a.htm#SQLRF00641 最后：http://download.oracle.com/docs/cd/B10501_01/server.920/a96540/functions57a.htm#83735

这是一个表格thistory的例子：

SQL> create table thistory (id,fk_user,state,timestamp)
  2  as
  3  select 1, 'Me', 'D', sysdate from dual union all
  4  select 2, 'Me', 'C', sysdate-1 from dual union all
  5  select 3, 'Me', 'B', sysdate-2 from dual union all
  6  select 4, 'Me', 'A', sysdate-3 from dual union all
  7  select 5, 'You', 'B', sysdate-11 from dual union all
  8  select 6, 'You', 'A', sysdate-12 from dual
  9  /

Table created.

SQL> exec dbms_stats.gather_table_stats(user,'thistory')

PL/SQL procedure successfully completed.

SQL> alter session set statistics_level = all
  2  /

Session altered.

SQL> set serveroutput off

首先使用子查询的pre-8i变体：

SQL> select *
  2    from thistory
  3   where timestamp =
  4         ( select max(timestamp)
  5             from thistory latest
  6            where fk_user = thistory.fk_user
  7         )
  8  /

        ID FK_USER S TIMESTAMP
---------- ------- - -------------------
         1 Me      D 19-05-2011 11:20:48
         5 You     B 08-05-2011 11:20:48

2 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'))
  2  /

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------
SQL_ID  306v8p42zdz34, child number 0
-------------------------------------
select *   from thistory  where timestamp =        ( select max(timestamp)            from thistory latest
       where fk_user = thistory.fk_user        )

Plan hash value: 2894184026

----------------------------------------------------------------------------------------------------------------------
| Id  | Operation            | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------
|*  1 |  HASH JOIN           |          |      1 |      2 |      2 |00:00:00.01 |       7 |  1155K|  1155K|  478K (0)|
|   2 |   VIEW               | VW_SQ_1  |      1 |      2 |      2 |00:00:00.01 |       3 |       |       |          |
|   3 |    HASH GROUP BY     |          |      1 |      2 |      2 |00:00:00.01 |       3 |       |       |          |
|   4 |     TABLE ACCESS FULL| THISTORY |      1 |      6 |      6 |00:00:00.01 |       3 |       |       |          |
|   5 |   TABLE ACCESS FULL  | THISTORY |      1 |      6 |      6 |00:00:00.01 |       4 |       |       |          |
----------------------------------------------------------------------------------------------------------------------

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

   1 - access("TIMESTAMP"="VW_COL_1" AND "FK_USER"="THISTORY"."FK_USER")


22 rows selected.

现在解析函数变体：

SQL> select *
  2    from ( select h.*
  3                , row_number() over (partition by h.fk_user order by timestamp desc) as rn
  4             from thistory h
  5         )
  6   where rn = 1
  7  /

        ID FK_USER S TIMESTAMP                   RN
---------- ------- - ------------------- ----------
         1 Me      D 19-05-2011 11:20:48          1
         5 You     B 08-05-2011 11:20:48          1

2 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'))
  2  /

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------
SQL_ID  b7zscht24wa2s, child number 0
-------------------------------------
select *   from ( select h.*               , row_number() over (partition by h.fk_user order by timestamp desc)
as rn            from thistory h        )  where rn = 1

Plan hash value: 2357375523

--------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------
|*  1 |  VIEW                    |          |      1 |      6 |      2 |00:00:00.01 |       3 |       |       |          |
|*  2 |   WINDOW SORT PUSHED RANK|          |      1 |      6 |      6 |00:00:00.01 |       3 |  9216 |  9216 | 8192  (0)|
|   3 |    TABLE ACCESS FULL     | THISTORY |      1 |      6 |      6 |00:00:00.01 |       3 |       |       |          |
--------------------------------------------------------------------------------------------------------------------------

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

   1 - filter("RN"=1)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY "H"."FK_USER" ORDER BY INTERNAL_FUNCTION("TIMESTAMP") DESC )<=1)


21 rows selected.

最后使用聚合函数的最有效变体：

SQL> select max(id) keep (dense_rank last order by timestamp) id
  2       , fk_user
  3       , max(state) keep (dense_rank last order by timestamp) state
  4       , max(timestamp)
  5    from thistory
  6   group by fk_user
  7  /

        ID FK_USER S MAX(TIMESTAMP)
---------- ------- - -------------------
         1 Me      D 19-05-2011 11:20:48
         5 You     B 08-05-2011 11:20:48

2 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'))
  2  /

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------
SQL_ID  9gcbnuf776q27, child number 0
-------------------------------------
select max(id) keep (dense_rank last order by timestamp) id      , fk_user      , max(state) keep
(dense_rank last order by timestamp) state      , max(timestamp)   from thistory  group by fk_user

Plan hash value: 76026975

--------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------
|   1 |  SORT GROUP BY     |          |      1 |      2 |      2 |00:00:00.01 |       3 |  9216 |  9216 | 8192  (0)|
|   2 |   TABLE ACCESS FULL| THISTORY |      1 |      6 |      6 |00:00:00.01 |       3 |       |       |          |
--------------------------------------------------------------------------------------------------------------------


14 rows selected.

如果你仔细观察计划，你会发现最后一个是最有效的。它不需要内联视图。

希望这有帮助。

的问候，
罗布。

获取历史表的每个外键的最后一个条目的正确方法是什么？

3 个答案: