Question

我有一个简单的查询：

SELECT distinct top 100  A.[Number]
FROM [Section] AS A
    LEFT JOIN [Customers] AS c  ON c.ARef = A.Number
    LEFT JOIN [Guides] AS G  ON G.CustomerId = c.CustomerId
    LEFT JOIN [Telephones] AS T  ON T.CustomerId = c.CustomerId
    LEFT JOIN [Epts] AS E  ON E.CustomerId = c.CustomerId
    LEFT JOIN [Emails] AS Em  ON Em.CustomerId = c.CustomerId
    LEFT JOIN [Addresses] AS Ad ON Ad.CustomerId=C.CustomerId
WHERE 
 A.SaloonId= 400
 AND (         
    A.Number= @term OR c.Surname = @term OR c.FirstName = @term 
    ----
    OR Ad.Postcode = @term
    OR G.CategoryRef= @term 
    OR T.PhoneNumber = @term 
    OR E.Code= @term 
    OR Em.EmailAddress = @term 
 )

where部分中包含的所有字段的索引碎片率都非常低。

如果我们对一个词执行查询，则需要花费超过20秒的时间，但是如果我删除最后一部分（“ ----”之后）中的任意行，则需要不到1秒的时间。

Answer 1

我将尝试将这些条件移至它们各自的JOIN，然后在WHERE中将其替换为OR Ad.CustomerId IS NOT NULL之类的条件。这样可以减少联接匹配，并减少必须评估联接条件的中间结果。

或者，由于大多数表中的任何数据均未真正返回结果中，因此我将考虑替换

...
LEFT JOIN [Guides] AS G  ON G.CustomerId = c.CustomerId 
... 
WHERE 
... 
OR G.CategoryRef = @term

与OR c.CustomerId IN (SELECT CustomerId FROM Guides AS G WHERE G.CategoryRef= @term)

如果A上的条件大大减少了C的数量，我什至会考虑使用OR EXISTS (SELECT * FROM Guides AS G WHERE G.CategoryRef= @term AND G.CustomerId = C.CustomerId)之类的相关子查询

条件过多时查询速度慢

1 个答案: