SQL查询速度非常慢，需要改进

我有一个select查询，需要改进以加快查询速度

SELECT 
    st.SigHistUID, st.RelationType, st2.RelationType, 
    case 
       when st.RelationType <> st2.RelationType 
          then 'N' 
          else st.RelationType 
    end as [NewRelationType],
    st.ToSigUID , sh.FullDescription, sh.SigHistUID AS ToSigHistUID
FROM 
    [Stackability] (nolock) st
INNER JOIN 
    [SignatureHistory] (nolock) sh ON sh.SigUID = st.ToSigUID
                                    AND sh.SigHistUID = (SELECT TOP 1 SigHistUID FROM [SignatureHistory] (nolock) tmp where tmp.SigUID = sh.SigUID ORDER BY SigHistUID DESC)
INNER JOIN 
    [SignatureHistory] (nolock) sh2 ON st.SigHistUID = sh2.SigHistUID
INNER JOIN 
    Stackability (nolock) st2 on st2.ToSigUID = sh2.SigUID and st2.SigHistUID = sh.SigHistUID
WHERE 
    st.SigHistUID < 1000 and
    st.RelationType <> st2.RelationType
ORDER BY 
    st.SigHistUID

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尝试排除前1个条件：

SELECT 
    st.SigHistUID, st.RelationType, st2.RelationType, 
    case 
       when st.RelationType <> st2.RelationType 
          then 'N' 
          else st.RelationType 
    end as [NewRelationType],
    st.ToSigUID , sh.FullDescription, sh.SigHistUID AS ToSigHistUID
FROM 
    [Stackability] (nolock) st
INNER JOIN 
    (select distinct siguid, max(SigHistUID) OVER(PARTITION BY siguid ) as  SigHistUID from [SignatureHistory] (nolock))  sh ON sh.SigUID = st.ToSigUID

INNER JOIN 
    [SignatureHistory] (nolock) sh2 ON st.SigHistUID = sh2.SigHistUID
INNER JOIN 
    Stackability (nolock) st2 on st2.ToSigUID = sh2.SigUID and st2.SigHistUID = sh.SigHistUID
WHERE 
    st.SigHistUID < 1000 and
    st.RelationType <> st2.RelationType
ORDER BY 
    st.SigHistUID

---

这信息太少了！桌子的结构是什么？在这些表上定义了什么类型的索引？？每个表格都有什么样的数据？至少你应该，你可以使用并分享你问题中的链接。此外，也许您可以通过查询找出性能问题。最后，在执行的查询中包含。根据SignatureHistory的大小，由于子查询结果未被索引，您的子查询可能会在sh连接上减慢速度。也许可以找到一种避免子查询的方法。旁注：我看到您在所有表上都使用nolock。我希望你理解这些含义。如果你不确定，非常感谢大家的回复。我还有最后一个问题，当stack.RelationType在mybatis中为NullStack\u recip.RelationType“-”时，有人知道如何写这个吗？