Question

我被要求生成一个由针对SQL Server数据库的相当复杂的SQL查询驱动的报告。由于报告的网站已经在使用Entity Framework 4.1，我想我会尝试使用EF和LINQ编写查询：

var q = from r in ctx.Responses
                    .Where(x => ctx.Responses.Where(u => u.UserId == x.UserId).Count() >= VALID_RESPONSES)
                    .GroupBy(x => new { x.User.AwardCity, x.Category.Label, x.ResponseText })
         orderby r.FirstOrDefault().User.AwardCity, r.FirstOrDefault().Category.Label, r.Count() descending
         select new
         {
             City = r.FirstOrDefault().User.AwardCity,
             Category = r.FirstOrDefault().Category.Label,
             Response = r.FirstOrDefault().ResponseText,
             Votes = r.Count()
         };

此查询统计投票，但仅限于已提交一定数量所需最低投票数的用户。

从性能角度来看，这种方法是一场彻底的灾难，因此我们切换到ADO.NET并且查询运行得非常快。我确实使用SQL事件探查器查看了LINQ生成的SQL，虽然它看起来像往常一样残酷，但我没有看到任何关于如何优化LINQ语句以提高效率的线索。

以下是直接的TSQL版本：

WITH ValidUsers(UserId)
AS
(
    SELECT UserId
    FROM Responses
    GROUP BY UserId
    HAVING COUNT(*) >= 103
)
SELECT d.AwardCity
    , c.Label
    , r.ResponseText
    , COUNT(*) AS Votes
FROM ValidUsers u
JOIN Responses r ON r.UserId = u.UserId
JOIN Categories c ON r.CategoryId = c.CategoryId
JOIN Demographics d ON r.UserId = d.Id
GROUP BY d.AwardCity, c.Label, r.ResponseText
ORDER BY d.AwardCity, s.SectionName, COUNT(*) DESC

我想知道的是：这个查询是否过于复杂，以至于EF和LINQ无法有效处理或者我错过了一招？

Answer 1

使用let来减少r.First（）的数量可能会提高性能。这可能还不够。

 var q = from r in ctx.Responses
                .Where()
                .GroupBy()
     let response = r.First()
     orderby response.User.AwardCity, response.Category.Label, r.Count() descending
     select new
     {
         City = response.User.AwardCity,
         Category = response.Category.Label,
         Response = response.ResponseText,
         Votes = r.Count()
     };

Answer 2

也许这个更改提高了性能，删除了where子句

中生成的嵌套sql select

首先获得每个用户的投票并将其放入Dictionary

var userVotes = ctx.Responses.GroupBy(x => x.UserId )
                             .ToDictionary(a => a.Key.UserId,  b => b.Count());

var cityQuery = ctx.Responses.ToList().Where(x => userVotes[x.UserId] >= VALID_RESPONSES)
               .GroupBy(x => new { x.User.AwardCity, x.Category.Label, x.ResponseText })
               .Select(r => new
                       {
                           City = r.First().User.AwardCity,
                           Category = r.First().Category.Label,
                           Response = r.First().ResponseText,
                           Votes = r.Count()
                       })
               .OrderByDescending(r => r.City, r.Category, r.Votes());

有没有办法优化这个LINQ to Entities查询？

2 个答案: