Question

基本上我在这个linq-to-nhibernate-produces-unnecessary-joins

中遇到了与Linq提供商相同的问题

List<Competitions> dtoCompetitions;
dtoCompetitions = (from compset in session.Query<FWBCompetitionSet>()
                    where compset.HeadLine == true 
                    && compset.A.B.CurrentSeason == true
                   select (new Competitions
                           {
                                       CompetitionSetID = compset.CompetitionSetID,
                                       Name = compset.Name,
                                       Description = compset.Description,
                                       Area = compset.Area,
                                       Type = compset.Type,
                                       CurrentSeason = compset.A.B.CurrentSeason,
                                       StartDate = compset.StartDate
                            }
                    )).ToList();

这导致其生成的SQL中的重复连接

SELECT fwbcompeti0_.competitionsetid AS col_0_0_,
   fwbcompeti0_.name             AS col_1_0_,
   fwbcompeti0_.DESCRIPTION      AS col_2_0_,
   fwbcompeti0_.area             AS col_3_0_,
   fwbcompeti0_.TYPE             AS col_4_0_,
   fwbseason3_.currentseason     AS col_5_0_,
   fwbcompeti0_.startdate        AS col_6_0_
FROM   fwbcompetitionset fwbcompeti0_
       INNER JOIN A fwbcompeti1_
         ON fwbcompeti0_.competitionseasonid = fwbcompeti1_.competitionseasonid
       INNER JOIN A fwbcompeti2_
         ON fwbcompeti0_.competitionseasonid = fwbcompeti2_.competitionseasonid
       INNER JOIN B fwbseason3_
         ON fwbcompeti2_.seasonid = fwbseason3_.seasonid
WHERE  fwbcompeti0_.headline = @p0
       AND fwbseason3_.currentseason = @p1

请注意这些连接，这些连接完全重复，也会影响我的SQL Server性能。

       INNER JOIN A fwbcompeti1_
         ON fwbcompeti0_.competitionseasonid = fwbcompeti1_.competitionseasonid
       INNER JOIN A fwbcompeti2_
         ON fwbcompeti0_.competitionseasonid = fwbcompeti2_.competitionseasonid

UPDATE1

在NHibernate 3.2中，这个LiNQ错误仍然有效，我找不到简单合理的Linq解决方案。所以我用QueryOver + JoinAlias + TransformUsing完成了这项工作，对我来说很完美。

FWBCompetitionSet compset = null;
FWBCompetitionSeason compseason = null;
FWBSeason season = null;
IList<Competitions> dtoCompetitions;
dtoCompetitions = session.QueryOver<FWBCompetitionSet>(() => compset)
.JoinAlias(() => compset.FWBCompetitionSeason, () => compseason)
.JoinAlias(() => compseason.FWBSeason, () => season)
.Where(() => compset.HeadLine == true)  
.And(() => season.CurrentSeason == true)
.SelectList(
list => list
.Select(c => c.CompetitionSetID).WithAlias(() => compset.CompetitionSetID)
.Select(c => c.Name).WithAlias(() => compset.Name)
.Select(c => c.Description).WithAlias(() => compset.Description)
.Select(c => c.Area).WithAlias(() => compset.Area)
.Select(c => c.Type).WithAlias(() => compset.Type)
.Select(c => season.CurrentSeason).WithAlias(() => season.CurrentSeason)
.Select(c => c.StartDate).WithAlias(() => compset.StartDate)
)
.TransformUsing(Transformers.AliasToBean<Competitions>())
.List<Competitions>();

Answer 1

又一个编辑：

我想我终于知道发生了什么。似乎LINQ to NHibernate提供程序在从目标到源表的关联中导航很困难，并且每次遇到这样的关联时都会生成单独的连接。

由于您没有提供映射，我使用了linq-to-nhibernate-produces-unnecessary-joins的映射。此模型具有文档，其中包含一个作业和许多 TranslationUnits 。每个 TranslationUnit 都有许多翻译实体。

当您尝试查找基于作业的翻译时，您将以相反的顺序遍历关联，并且LINQ提供程序生成多个联接：一个用于翻译 - ＆gt; TranslationUnit和TranslationUnit to Document。

此查询将生成冗余连接：

session.Query<TmTranslation>()
          .Where(x => x.TranslationUnit.Document.Job == job)
          .OrderBy(x => x.Id)
          .ToList();

如果您将导航订单反转为Document - ＆gt; TranslationUnit - ＆gt;翻译，你得到一个不产生任何冗余连接的查询：

var items=(from doc in session.Query<Document>()
        from tu in doc.TranslationUnits
            from translation in tu.Translations
       where doc.Job ==job                        
       orderby translation.Id
       select translation).ToList();

鉴于这种怪癖，QueryOver似乎是一个更好的选择。

上一页修改：

我怀疑罪魁祸首是 compset.A.B.CurrentSeason 。第一个连接表（fwbcompeti1_）返回 A .B，而接下来的两个（fwbcompeti2_和fwbseason3_）用于返回A. B 。 LINQ to NHibernate提供程序似乎没有猜到A在其他任何地方都没有使用，并且无法将其从生成的语句中删除。

尝试用选择<中的 CurrentSeason = true 替换 CurrentSeason = compset.ABCurrentSeason 来帮助优化器一点/ em>，因为where语句只返回CurrentSeason == true的项目。

编辑：我的意思是更改这样的查询：

List<Competitions> dtoCompetitions; dtoCompetitions = (from compset in session.Query<FWBCompetitionSet>() where compset.HeadLine == true && compset.A.B.CurrentSeason == true select (new Competitions { CompetitionSetID = compset.CompetitionSetID, Name = compset.Name, Description = compset.Description, Area = compset.Area, Type = compset.Type, CurrentSeason = true, StartDate = compset.StartDate } )).ToList();

我只需将值compset.A.B.CurrentSeason替换为true

在NHibernate中使用Linq时重复和不必要的连接

1 个答案: