SQL Server UNION ALL合併連接（連接）太慢

Question

我有一個選擇查詢，它在具有相同結構（列和主鍵，它們具有不同非聚簇索引）的兩個表上使用UNION ALL關鍵字。這兩個表格包含3900萬行，其中一個是100萬，另一個是3800萬。當僅在table1上運行具有一百萬行的查詢時，大約需要0.2秒，在table2上，根據DB的壓力，我們有不同的情況，最多需要0.5到1.2秒。

實際上，爲了顯示我需要聯合這兩個表，但問題是聯合查詢需要高達8秒才能運行。當看執行計劃時，最重的操作是Merge Join (Concatenation)，成本爲91％，我有點擔心，因爲我正在運行的WHERE子句從table1中選擇51個條目，並從table2（大表中）選擇0個條目。

我無法理解它，我一直試圖找到解決我的問題的最後兩天現在，我發現的所有或者是UNION而不是UNION ALL或不需要的子句正在就位GROUP BY或LEFT/INNER JOINS。查詢也進行分頁，使用這個命令ORDER BY [Id] DESC OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY;，所有測試（在單個表格上和使用UNION ALL）都使用分頁（OFFSET和FETCH NEXT）關鍵字進行。

如果需要，我可以提供表詳細信息和查詢詳細信息。它是一個簡單的select查詢，其中2 INNER JOINS和2 LEFT JOINS所有連接的表包含非常少量的數據（範圍從50個條目到20K個條目）。

---

這裏的查詢

SELECT * 
FROM  (SELECT tr.Id, 
     tr.Amount, 
     tr.TypeId, 
     t.Name AS [Type], 
     tr.Date, 
     tr.ExternalKey, 
     tr.ExternalDescription, 
     tr.GameId, 
     tr.GameProviderId, 
     gp.Name AS GameProvider, 
     u.Username, 
     u.Pincode, 
     gp.Name, 
     g.GameName, 
     u.OperatorId, 
     tr.BalanceBefore, 
     tr.BalanceAfter, 
     tr.UserId 
FROM ( 
     SELECT * 
       FROM  dbo.ActiveTransactions at 
       WHERE (1 = 1) 
     AND ([Date] >= '2017-07-17 20:00:00') 
     AND ([TypeId] != 10) 
     AND ([UserId] = 29041) 
       UNION ALL 
       SELECT * 
       FROM  dbo.TransactionHistory th --WITH(INDEX(IX_TransactionHistory_DateType_UserId)) 
       WHERE (1 = 1) 
     AND ([Date] >= '2017-07-17 20:00:00') 
     AND ([TypeId] != 10) 
     AND ([UserId] = 29041) 
      ) AS tr 

INNER JOIN dbo.Users u ON tr.UserId = u.Id 
LEFT JOIN dbo.GameProviders gp ON tr.GameProviderId = gp.Id 
LEFT JOIN dbo.Games g ON tr.GameId = g.GameId AND tr.GameProviderId = g.ProviderId 
INNER JOIN dbo.Types t ON tr.TypeId = t.Id ) AS t 
ORDER BY [Id] DESC OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY; 

Answer 1

這是一個受過教育的猜測：沒有索引或執行計劃的什麼發動機實際上是陷入了知識。

考慮：在連接/過濾器之後而不是之前執行聯合：您必須重複聯接，但是您可能會在聯合集處理中損失一些索引效率。

在這種情況下，我創建了兩個CTE，然後將它們聯合起來。

因爲我無法真正測試這個，我可能有一些語法錯誤。

WITH cte1 AS 
(SELECT tr.Id, 
     tr.Amount, 
     tr.TypeId, 
     t.Name AS [Type], 
     tr.Date, 
     tr.ExternalKey, 
     tr.ExternalDescription, 
     tr.GameId, 
     tr.GameProviderId, 
     gp.Name AS GameProvider, 
     u.Username, 
     u.Pincode, 
     gp.Name, 
     g.GameName, 
     u.OperatorId, 
     tr.BalanceBefore, 
     tr.BalanceAfter, 
     tr.UserId 
FROM ( 
     SELECT * 
       FROM  dbo.ActiveTransactions at 
       WHERE (1 = 1) 
     AND ([Date] >= '2017-07-17 20:00:00') 
     AND ([TypeId] != 10) 
     AND ([UserId] = 29041)) AS tr 
INNER JOIN dbo.Users u ON tr.UserId = u.Id 
LEFT JOIN dbo.GameProviders gp ON tr.GameProviderId = gp.Id 
LEFT JOIN dbo.Games g ON tr.GameId = g.GameId AND tr.GameProviderId = g.ProviderId 
INNER JOIN dbo.Types t ON tr.TypeId = t.Id ) AS t), 
CTE2 as (
SELECT tr.Id, 
     tr.Amount, 
     tr.TypeId, 
     t.Name AS [Type], 
     tr.Date, 
     tr.ExternalKey, 
     tr.ExternalDescription, 
     tr.GameId, 
     tr.GameProviderId, 
     gp.Name AS GameProvider, 
     u.Username, 
     u.Pincode, 
     gp.Name, 
     g.GameName, 
     u.OperatorId, 
     tr.BalanceBefore, 
     tr.BalanceAfter, 
     tr.UserId 

FROM (SELECT * 
     FROM dbo.TransactionHistory th --WITH(INDEX(IX_TransactionHistory_DateType_UserId)) 
     WHERE (1 = 1) 
     AND ([Date] >= '2017-07-17 20:00:00') 
     AND ([TypeId] != 10) 
     AND ([UserId] = 29041) as tr 
INNER JOIN dbo.Users u ON tr.UserId = u.Id 
LEFT JOIN dbo.GameProviders gp ON tr.GameProviderId = gp.Id 
LEFT JOIN dbo.Games g ON tr.GameId = g.GameId AND tr.GameProviderId = g.ProviderId 
INNER JOIN dbo.Types t ON tr.TypeId = t.Id) AS t 
) 
SELECT * from CTE1 
UNION ALL 
SELECT * from CTE2 
ORDER BY [Id] DESC OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY;