Question

我编写了一个lambda表达式，它产生了预期的结果，但它生成了一个绝对庞大的sql查询，并且性能不佳。请参阅最底层的io /时间统计信息。

有没有其他方法可以实现以下查询？

select distinct(searchterms) as SearchTerms, max(totalresults) FROM cmsSearchLog 
where totalresults != 0 and searchterms like 'de%' group by searchterms 
order by max(totalresults) desc

c＃code snippets：

// current lamda expression; has bad performance compared to above query
List<SearchTerm> existingSearchTerms1 = context.cmsSearchLogs.Where(oq =>
context.cmsSearchLogs.Where(q =>
q.SearchTerms.ToLower().Contains(terms.ToLower()) && q.TotalResults != 0)
.Select(s => s.SearchTerms)
.Distinct()
.Contains(oq.SearchTerms))
.Select(a => new { a.SearchTerms, a.TotalResults })
.GroupBy(gb => gb.SearchTerms)
.OrderByDescending(ob => ob.Max(m => m.TotalResults))
.Select(s => new SearchTerm()
    {
        SearchTerms = s.FirstOrDefault().SearchTerms,
        TotalResults = s.FirstOrDefault().TotalResults
    }
)
.ToList();

// get the suggestions back as a list of strings
List<string> suggestions = Enumerable.Range(0, 
  existingSearchTerms1.Count())
  .Select(x => existingSearchTerms1.ElementAt(x).SearchTerms).ToList();

这是保存查询结果的私有类

private class SearchTerm
{
    public string SearchTerms { get; set; }
    public int TotalResults { get; set; }
}

lambda表达式生成的sql很大：

SELECT 
[Project13].[C2] AS [C1], 
[Project13].[C3] AS [C2], 
[Project13].[C4] AS [C3]
FROM ( SELECT 
    [Project12].[C1] AS [C1], 
    1 AS [C2], 
    [Project12].[C2] AS [C3], 
    [Project12].[C3] AS [C4]
    FROM ( SELECT 
        [Project8].[C1] AS [C1], 
        [Project8].[C2] AS [C2], 
        (SELECT TOP (1) 
            [Extent5].[TotalResults] AS [TotalResults]
            FROM [dbo].[cmsSearchLog] AS [Extent5]
            WHERE ( EXISTS (SELECT 1 AS [C1]                    
               FROM ( SELECT DISTINCT 
            [Extent6].[SearchTerms] AS [SearchTerms]
            FROM [dbo].[cmsSearchLog] AS [Extent6]
            WHERE (( CAST(CHARINDEX(LOWER('dew'), 
                             LOWER([Extent6].[SearchTerms])) AS int)) > 0) 
                             AND (0 <> [Extent6].[TotalResults])
                )  AS [Distinct3]
            WHERE [Distinct3].[SearchTerms] = [Extent5].[SearchTerms]
            )) AND ([Project8].[SearchTerms] = [Extent5].[SearchTerms])) 
                                AS [C3]
        FROM ( SELECT 
           [Project7].[C1] AS [C1], 
           [Project7].[SearchTerms] AS [SearchTerms], 
           [Project7].[C2] AS [C2]
           FROM ( SELECT 
              [Project3].[C1] AS [C1], 
              [Project3].[SearchTerms] AS [SearchTerms], 
              (SELECT TOP (1) 
              [Extent3].[SearchTerms] AS [SearchTerms]
              FROM [dbo].[cmsSearchLog] AS [Extent3]
              WHERE ( EXISTS (SELECT 1 AS [C1] FROM ( SELECT DISTINCT 
            [Extent4].[SearchTerms] AS [SearchTerms]
            FROM [dbo].[cmsSearchLog] AS [Extent4]
            WHERE (( CAST(CHARINDEX(LOWER('dew'), 
                             LOWER([Extent4].[SearchTerms])) AS int)) > 0) 
                             AND (0 <> [Extent4].[TotalResults]))  AS [Distinct2] 
           WHERE [Distinct2].[SearchTerms] = [Extent3].[SearchTerms]
               )) AND ([Project3].[SearchTerms] = [Extent3].[SearchTerms])) AS [C2]
                FROM ( SELECT 
                  [GroupBy1].[A1] AS [C1], 
                  [GroupBy1].[K1] AS [SearchTerms]
                  FROM ( SELECT 
                   [Extent1].[SearchTerms] AS [K1], 
                   MAX([Extent1].[TotalResults]) AS [A1]
                   FROM [dbo].[cmsSearchLog] AS [Extent1]
                   WHERE EXISTS (SELECT 1 AS [C1]
                FROM ( SELECT DISTINCT [Extent2].[SearchTerms]
                  AS [SearchTerms] FROM [dbo].[cmsSearchLog] AS [Extent2]
                        WHERE (( CAST(CHARINDEX(LOWER('dew'),
                                      LOWER([Extent2].[SearchTerms])) AS int)) > 0)
                                       AND (0 <> [Extent2].[TotalResults]))  AS [Distinct1]
                                       WHERE [Distinct1].[SearchTerms] = [Extent1].[SearchTerms])
                 GROUP BY [Extent1].[SearchTerms])  AS [GroupBy1]
                )  AS [Project3]
            )  AS [Project7]
        )  AS [Project8]
    )  AS [Project12]
)  AS [Project13]
ORDER BY [Project13].[C1] ASC

我用io执行了两个查询并打开了时间统计信息，结果如下。（注意：lambda生成的查询是第一个，我的手写查询第二个）所以这证实了我怀疑生成的查询与我实际想要的查询相比表现得非常糟糕。

(8 row(s) affected)
Table 'cmsSearchLog'. Scan count 6, logical reads 106, physical reads 0, 
read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 1 ms.

(7 row(s) affected)
Table 'cmsSearchLog'. Scan count 1, logical reads 5, physical reads 0, 
read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.

Answer 1

尝试此查询而不是您当前的LINQ查询：

var query = from x in context.cmsSearchLog
            where totalresults != 0 &&
                  searchterms.BeginsWith("de")
            group x by x.searchterms into terms
            select new {
                           SearchTerms = terms.Key(),
                           TotalResults = terms.Max(t => t.totalresults)
                       };

我还没有对它进行测试，但我认为它会生成一个非常有效的查询并返回所需的结果。

Answer 2

LINQ转换（无论是LINQ to SQL，实体框架等）是关于高效开发。它允许（理论上）更可读，可维护的代码，并减少由于胖指法等引起的运行时数据库错误的可能性.LINQ 不关于性能。 LINQ通常提供“足够好”的性能，但它永远不会像手动编码的查询或存储过程那样击败更接近金属的东西。

那就是说，你的查询返回不同的行数，因此其中一个（或两个）是错误的;第一个查询产生8行，而第二个产生7行。你不能很好地比较提供不同结果的查询！

Answer 3

对于复杂或性能密集型查询，不要觉得您无法创建视图或用户定义的函数，而是映射到该函数。在这种情况下，您甚至可以使用存储过程并映射到该过程。

Answer 4

首先，您需要知道lambda表达式方法不适用于此类查询。但是，如果您对hack没问题，请创建一个使用以下内容的视图：

select distinct searchTerm, max(totalresults) 
from cmsSearchLog 
group by searchterms 
order by max(totalresults) desc

然后使用你的lambda表达式来做过滤部分

Answer 5

为什么不让数据库处理此查询的工作并将结果直接转储到SearchTerm类中？如果需要查找特定术语，可以参数化该过程。在您提供的示例中，您可以通过索引searchterms列来进一步提高性能，因为where子句中的通配符引用了列值文本的尾部。此外，由于您在searchterms上进行分组，因此无需在该列上调用distinct（这可能会也可能不会提高性能，具体取决于系统选择执行的查询计划）。

从lambda表达式生成sql server查询的替代方法

5 个答案: