爲集合中的每個類別有效選擇頂行

Question

我需要從已知集合中選擇每個類別的頂行（與this question有點類似）。問題是，如何使這個查詢在大量的行上有效。

例如，我們創建一個表格，在幾個地方存儲溫度記錄。

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 

現在我需要獲得最新的記錄每個地區1，2，3

這種方式是有效的，但不能很好地（而且看上去髒）。

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 

以下看起來更好但效率低得多（根據優化器，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 

的問題是，在#T執行聚集索引掃描和300個檢索行，分類，編號，然後過濾，只留下3行後者查詢執行計劃中。對於前一個查詢，三次獲取一行。

有沒有辦法有效地執行查詢沒有大量的聯合？

Answer 1

我裝100,000行（這仍然不是足以減慢速度），嘗試了老式的方式：

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 

並得到了很多相同的結果。

然後我扭轉了索引中的列的順序，以

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

，並在所有的查詢，減少了頁面讀取和指數尋求而不是掃描。

如果優化是你的目標，你可以修改索引，我修改了主鍵，或者添加了一個覆蓋索引。

Answer 2

不只是看執行計劃還看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'。掃描計數3，邏輯讀取6，物理讀取0，預讀讀取0，lob邏輯讀取0，lob物理讀取0，lob預讀讀取0. 表'＃t000000000B99'。掃描計數1，邏輯讀取6次，物理讀取0，預讀0，lob邏輯讀取0，lob物理讀取0次，lob預讀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 

對我來說，有沒有真正的區別2種方法，加載了更多的數據，當您將它下降到40％和60％這兩個查詢添加訂單也再次比較

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

只是爲了記錄，另一個選項使用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到臨時表或表變量沒有太大的區別。