Question

我在EF Core 2.0中有这样的linq查询，它按原样工作，但是当我升级到EF Core 3.0时，它总是超时。我在query = query.Where(x => x.Questions);中发现了问题。
我的问题是我想返回仅带Take（10）或带过滤条件的课程。条件仅显示特定范围而不是所有问题。

var query = _courseRepository.Table;
query = query.Where(x => x.Id == id);
query = query.Include(x => x.Questions);
query = query.Include(x => x.CourseYear);
query = query.Include(x => x.CourseSubject);
query = query.Include(x => x.Instructors).ThenInclude(y => y.User);
query = query.Include(x => x.Instructors).ThenInclude(y => y.Course);
query = query.Include(x => x.Instructors).ThenInclude(y => y.CourseClass);
query = query.Include(x => x.CourseSections);
query = query.Include(x => x.CourseSections).ThenInclude(y => y.Lessons);
query = query.Include(x => x.CourseClasses);
query = query.Include(x => x.UserCourses).ThenInclude(y => y.User);
var result = query.FirstOrDefault();

Answer 1

EFCore 3.0更改了使用.Include()生成的查询，您遇到了笛卡尔爆炸问题;

具体来说，现在文档中有以下红色警告：

注意

从3.0.0版开始，每个Include都会导致一个附加的JOIN   添加到关系提供程序产生的SQL查询中，而   早期版本生成了其他SQL查询。这个可以   显着改变查询的性能，以达到更好或   更差。特别是，LINQ查询中的查询数量极高   包含运算符可能需要细分为多个单独的   LINQ查询是为了避免笛卡尔爆炸问题。

解决方案是现在根据文档执行多个查询。

它的超级不幸的加载实体图（对高度规范化的数据通用）非常糟糕，但这是EF的当前状态。

请参阅：加载Related Data并滚动直到看到红色。

Answer 2

var query = _courseRepository.Table
.Include(x => x.Questions)
.Include(x => x.CourseClasses)
.Include(x => x.CourseYear)
.Include(x => x.CourseSubject);

var course = await query.FirstOrDefaultAsync(x => x.Id == id);

query.Include(x => x.Instructors).ThenInclude(y => y.User).SelectMany(a => a.Instructors).Load();
query.Include(x => x.Instructors).ThenInclude(y => y.Course).SelectMany(a => a.Instructors).Load();
query.Include(x => x.Instructors).ThenInclude(y => y.CourseClass).SelectMany(a => a.Instructors).Load();

query.Include(x => x.CourseSections).ThenInclude(y => y.Lessons).SelectMany(a => a.CourseSections).Load();

query.Include(x => x.UserCourses).ThenInclude(y => y.User).SelectMany(a => a.UserCourses).Load();

EF Core 3.0 .Include不能按预期工作，并且超级慢

2 个答案: