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我使用 Linq2Sql 作为 ORM 并遇到以下问题：我有 2 个表，让我们称它们为客户端（id、名称）和 client_action（id、clientid、日期）。'client' 表包含客户端，client_action 包含用户操作的日志，我只需要其中的第一个。

有一个 linq2sql 查询（带有分页和排序），导致以下 sql：

SELECT TOP (10) /* columns here */
FROM (
    SELECT /* columns here */
    FROM [dbo].[client] AS [t0]
    CROSS APPLY ((
            SELECT NULL AS [EMPTY]
            ) AS [t8]
        OUTER APPLY (
            SELECT [t9].[source]
            FROM [action] AS [t9]
            INNER JOIN (
                SELECT MIN([t10].[actionID]) AS [value], [t10].[ClientID]
                FROM [client_action] AS [t10]
                GROUP BY [t10].[clientID]
                ) AS [t11] ON [t9].[ID] = [t11].[value]
            WHERE [t0].[ID] = [t11].[ClientID]
            ) AS [t12])
    ) AS [t17]
WHERE /* filter for fields from the client table */
ORDER BY [t17].[date_created] DESC

该查询中有一个逻辑错误，我更改了它。现在生成的sql如下所示：

SELECT TOP (10) /* columns here */
FROM (
    SELECT /* columns here */
    FROM (
        SELECT /* columns here */
        FROM [dbo].[client] AS [t0]
        CROSS APPLY ((
                SELECT NULL AS [EMPTY]
                ) AS [t3]
            OUTER APPLY (
                SELECT [t7].[value]
                FROM (
                    SELECT [t5].[ClientID], (
                        SELECT TOP (1) [t6].[ID]
                        FROM [client_action] AS [t6]
                        WHERE [t5].[clientID] = [t6].[clientID]
                        ORDER BY [t6].[date_created]
                        ) AS [value]
                    FROM (
                        SELECT [t4].[clientID]
                        FROM [client_action] AS [t4]
                        GROUP BY [t4].[clientID]
                        ) AS [t5]
                    ) AS [t7]
                WHERE [t0].[ID] = [t7].[clientID]
                ) AS [t8])
        LEFT OUTER JOIN [client_action] AS [t9] ON [t8].[value] = [t9].[ID]
        ) AS [t26]
    ) AS [t27]
WHERE /* filter for fields from client table */
ORDER BY [t27].[date_created] DESC

并且出现了性能问题。我使用“SET STATISTICS IO ON”执行了这两个查询并得到了有趣的结果：

  • 对于第一个查询：
    • 客户表被过滤
    • 申请了“前 10 名”
    • 'cross apply' 应用于结果 10 条记录
  • 对于第二个查询：
    • 客户表被过滤（找到大约 10K 条记录）
    • 'cross apply' 申请了那 10K 条记录
    • 申请了“前 10 名”

所以它使第二个查询工作得慢得多。有没有办法“调整”查询，以便尽可能在过滤后应用“交叉应用”？

问题是那里有 2 个“交叉应用”和更多的连接，这一切都使它工作得非常慢。我创建了可以提高查询速度的索引，但它仍然慢到令人讨厌。

帮帮帮帮 。_

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