Sql 在（）中使用筛选器WHERE字段时从大表中删除花费的时间太长

我有一个奇怪的问题，我不太明白。有一个很大的表，我们称之为LT，有一个主键ID。我有一个DELETE语句来删除重复项，如下所示：

DELETE FROM LT
WHERE ID IN ( 
    SELECT l.ID FROM ( 
        SELECT Field1, Field2, COUNT(*) as total, MIN(ID) LowestID FROM LT (NOLOCK) WHERE Field1 = @fieldOneParam 
        GROUP BY Field1, Field2
        HAVING COUNT(*) > 1 
    ) a 
    INNER JOIN LT l (NOLOCK) on l.Field1 = a.Field1 And l.Field2 = a.Field2 And l.ID > a.LowestID 
)

如果嵌套选择中有结果，则查询运行相对较快。但是，当此子查询：

SELECT l.ID FROM ( 
    SELECT Field1, Field2, COUNT(*) as total, MIN(ID) LowestID FROM LT (NOLOCK) WHERE Field1 = @fieldOneParam 
    GROUP BY Field1, Field2
    HAVING COUNT(*) > 1 
) a 
INNER JOIN LT l (NOLOCK) on l.Field1 = a.Field1 And l.Field2 = a.Field2 And l.ID > a.LowestID 

DELETE FROM LT
WHERE LT.ID > ( 
    SELECT MIN(sub.ID)
    FROM LT sub
    WHERE sub.Field1 = @fieldOneParam 
      AND sub.Field1 = LT.Field1 AND sub.Field2 = LT.Field2
    HAVING COUNT(*) > 1     
)

返回一个空集，整个语句将永远运行。我检查了估计的执行计划，它充满了表假脱机、嵌套循环，并查找表中的每个索引，这些索引很多。不是针对子查询，而是针对删除本身

为什么会这样？它不应该只对它拥有的ID字段使用聚集索引吗？查询不应该在几秒钟内运行，因为在。。。子句是否为空

---

谢谢

这并不能回答您的问题，但这里有一种替代方法，可以删除重复项，同时使用和保持每个字段1、字段2的最低id：

原始查询中的NOLOCK提示可能是导致问题的原因

---

而不是对性能问题的具体回答，考虑一级相关子查询：

SELECT l.ID FROM ( 
    SELECT Field1, Field2, COUNT(*) as total, MIN(ID) LowestID FROM LT (NOLOCK) WHERE Field1 = @fieldOneParam 
    GROUP BY Field1, Field2
    HAVING COUNT(*) > 1 
) a 
INNER JOIN LT l (NOLOCK) on l.Field1 = a.Field1 And l.Field2 = a.Field2 And l.ID > a.LowestID 

DELETE FROM LT
WHERE LT.ID > ( 
    SELECT MIN(sub.ID)
    FROM LT sub
    WHERE sub.Field1 = @fieldOneParam 
      AND sub.Field1 = LT.Field1 AND sub.Field2 = LT.Field2
    HAVING COUNT(*) > 1     
)

---

我以前也注意到过类似的行为

如果将ID保存到临时表中，则可以从临时表中ID所在的主表中删除

因此，在您的代码中，您可以将其修改为：

DECLARE @temp TABLE (ID int PRIMARY KEY)

SELECT l.ID 
INTO @temp
FROM ( 
        SELECT Field1, Field2, COUNT(*) as total, MIN(ID) LowestID FROM LT (NOLOCK) WHERE Field1 = @fieldOneParam 
        GROUP BY Field1, Field2
        HAVING COUNT(*) > 1 
    ) a 
    INNER JOIN LT l (NOLOCK) on l.Field1 = a.Field1 And l.Field2 = a.Field2 And l.ID > a.LowestID

DELETE FROM LT
WHERE LT.ID IN (SELECT ID 
                  FROM @temp)

---

如果将子查询替换为in select 1 from LT，其中1=0，则查询是否永远运行？