为集合中的每个类别有效选择顶行

Question

我需要从已知集合中选择每个类别的顶行（与this question有点类似）。问题是，如何使这个查询在大量的行上有效。

例如，我们创建一个表格，在几个地方存储温度记录。

CREATE TABLE #t (
    placeId int, 
    ts datetime, 
    temp int, 
    PRIMARY KEY (ts, placeId) 
) 

-- insert some sample data 

SET NOCOUNT ON 

DECLARE @n int, @ts datetime 
SELECT @n = 1000, @ts = '2000-01-01' 

WHILE (@n>0) BEGIN 
    INSERT INTO #t VALUES (@n % 10, @ts, @n % 37) 
    IF (@n % 10 = 0) SET @ts = DATEADD(hour, 1, @ts) 
    SET @n = @n - 1 
END 

现在我需要获得最新的记录每个地区1，2，3

这种方式是有效的，但不能很好地（而且看上去脏）。

SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 1 
    ORDER BY ts DESC 
) t1 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 2 
    ORDER BY ts DESC 
) t2 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 3 
    ORDER BY ts DESC 
) t3 

以下看起来更好但效率低得多（根据优化器，30％vs 70％）。

SELECT placeId, ts, temp FROM (
    SELECT placeId, ts, temp, ROW_NUMBER() OVER (PARTITION BY placeId ORDER BY ts DESC) rownum 
    FROM #t 
    WHERE placeId IN (1, 2, 3) 
) t 
WHERE rownum = 1 

的问题是，在#T执行聚集索引扫描和300个检索行，分类，编号，然后过滤，只留下3行后者查询执行计划中。对于前一个查询，三次获取一行。

有没有办法有效地执行查询没有大量的联合？

Answer 1

我装100,000行（这仍然不是足以减慢速度），尝试了老式的方式：

select t.* 
from #t t 
    inner join (select placeId, max(ts) ts 
       from #t 
       where placeId in (1,2,3) 
       group by placeId) xx 
    on xx.placeId = t.placeId 
    and xx.ts = t.ts 

并得到了很多相同的结果。

然后我扭转了索引中的列的顺序，以

CREATE TABLE #t ( 
    placeId int, 
    ts datetime, 
    temp int, 
    PRIMARY KEY (placeId, ts) 
) 

，并在所有的查询，减少了页面读取和指数寻求而不是扫描。

如果优化是你的目标，你可以修改索引，我修改了主键，或者添加了一个覆盖索引。

Answer 2

不只是看执行计划还看statistics io和statistics time

set statistics io on 
go 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 1 
    ORDER BY ts DESC 
) t1 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 2 
    ORDER BY ts DESC 
) t2 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 3 
    ORDER BY ts DESC 
) t3 

SELECT placeId, temp FROM (
    SELECT placeId, ts, temp, ROW_NUMBER() OVER (PARTITION BY placeId ORDER BY ts DESC) rownum 
    FROM #t 
    WHERE placeId IN (1, 2, 3) 
) t 
WHERE rownum = 1 

set statistics io off 
go 

表 '＃t000000000B99'。扫描计数3，逻辑读取6，物理读取0，预读读取0，lob逻辑读取0，lob物理读取0，lob预读读取0. 表'＃t000000000B99'。扫描计数1，逻辑读取6次，物理读取0，预读0，lob逻辑读取0，lob物理读取0次，lob预读0

set statistics time on 
go 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 1 
    ORDER BY ts DESC 
) t1 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 2 
    ORDER BY ts DESC 
) t2 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 3 
    ORDER BY ts DESC 
) t3 

SELECT placeId, temp FROM (
    SELECT placeId, ts, temp, ROW_NUMBER() OVER (PARTITION BY placeId ORDER BY ts DESC) rownum 
    FROM #t 
    WHERE placeId IN (1, 2, 3) 
) t 
WHERE rownum = 1 

set statistics time on 
go 

对我来说，有没有真正的区别2种方法，加载了更多的数据，当您将它下降到40％和60％这两个查询添加订单也再次比较

SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 1 
    ORDER BY ts DESC 
) t1 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 2 
    ORDER BY ts DESC 
) t2 
UNION ALL 
SELECT * FROM (
    SELECT TOP 1 placeId, temp 
    FROM #t 
    WHERE placeId = 3 
    ORDER BY ts DESC 
) t3 
ORDER BY placeId 

SELECT placeId, temp FROM (
    SELECT placeId, temp, ROW_NUMBER() OVER (PARTITION BY placeId ORDER BY ts DESC) rownum 
    FROM #t 
    WHERE placeId IN (1, 2, 3) 
) t 
WHERE rownum = 1 
ORDER BY placeId 

Answer 3

只是为了记录，另一个选项使用CROSS APPLY。
在我的配置上，它的性能比以前提到的要好。

SELECT * 
FROM (VALUES (1),(2),(3)) t (placeId) 
CROSS APPLY (
    SELECT TOP 1 ts, temp 
    FROM #t 
    WHERE placeId = t.placeId 
    ORDER BY ts DESC 
) tt 

我猜，VALUES可能被chaged到临时表或表变量没有太大的区别。