SQL Server OR 运算符导致大量循环连接

Question

下面的查询非常慢（运行超过一分钟），我已将问题缩小到OR操作员 ( ...OR (EXISTS (SELECT...)。

我使用实时执行来验证 OR 语句的表之间是否存在嵌套循环连接，然后将记录连接回EmailTable执行计划中的 。

基本上，EmailTable正在被探查两次。

如果我添加提示OPTION (MERGE JOIN)，查询将在一秒钟内完成。

请告诉我如何重写此查询，以便优化器默认选择更好的计划。

EmailTable并且TeamMembers在 上有聚集索引INS_ID。表上的统计数据经常更新。

DECLARE @a INT
    ,@b BIT
    ,@c INT
    ,@d INT
    ,@e INT;

SELECT [XYZ].[CNT] AS [C]
FROM (
    SELECT COUNT(1) AS [CNT]
    FROM [dbo].[EmailTable] AS [table1]
    WHERE ([table1].[INS_ID] = @a)
        AND ([table1].[ACTIVE] = 1)
        AND ([table1].[QUEUED_TO_SEND] = @b)
        AND ([table1].[OWNER_USER_ID] <> @c)
        AND (
            ([table1].[OWNER_USER_ID] IN (- 1))
            OR (N'Allusers' = [table1].[VISIBLE_TO])
            OR ([table1].[OWNER_USER_ID] = @d)
            OR (
                EXISTS (
                    SELECT 1 AS [C]
                    FROM [dbo].[TeamMembers] AS [table2]
                    WHERE ([table1].[INS_ID] = [table2].[INS_ID])
                        AND ([table1].[VISIBLE_TEAM_ID] = [table2].[TEAM_ID])
                        AND ([table2].[INS_ID] = [table1].[INS_ID])
                        AND ([table2].[MEMBER_USER_ID] = @d)
                        AND ([table2].[TEAM_ID] = [table1].[VISIBLE_TEAM_ID])
                    )
                )
            )
    ) AS [XYZ];

Answer 1

两个表之间存在一些冗余 JOIN，并且在嵌套查询中使用太多OR条件有时会压垮 SQL 查询优化器。

清理和组织代码后，我提出了以下（希望是等效的）替代方案：

DECLARE @a INT
    ,@b BIT
    ,@c INT
    ,@d INT
    ,@e INT;

SELECT SUM(CNT) AS [C]
FROM (
    SELECT COUNT(1) AS [CNT]
    FROM [dbo].[EmailTable]
    WHERE 1 = 1
        AND [ACTIVE] = 1
        AND [INS_ID] = @a
        AND [QUEUED_TO_SEND] = @b
        AND [OWNER_USER_ID] <> @c
        AND (
            [OWNER_USER_ID] IN (- 1)
            OR N'Allusers' = [VISIBLE_TO]
            OR [OWNER_USER_ID] = @d
            )

    UNION ALL

    SELECT COUNT(1) AS [CNT]
    FROM [dbo].[EmailTable] AS [table1]
    WHERE 1 = 1
        AND [table1].[ACTIVE] = 1
        AND [table1].[INS_ID] = @a
        AND [table1].[QUEUED_TO_SEND] = @b
        AND [table1].[OWNER_USER_ID] <> @c
        AND EXISTS (
            SELECT 1 AS [C]
            FROM [dbo].[TeamMembers] AS [table2]
            WHERE [table2].[INS_ID] = [table1].[INS_ID]
                AND [table2].[TEAM_ID] = [table1].[VISIBLE_TEAM_ID]
                AND [table2].[MEMBER_USER_ID] = @d
            )
    ) XYZ
OPTION(RECOMPILE);

让我们知道它是如何工作的，并可能按照一些评论者的要求发布实际执行计划。