使用Entity Framework和LINQ选择和过滤数据

Question

Suppose I have the following query:

 context.Orders.Where(predicate).Select(x => new { propA = x.PropA, propB = x.PropB}).ToList();

Does this perform both the where and select functions at the DB/ORM level, returning only the data that satisfy both expressions, or would it return all the results that satisfy the predicate and then perform the select on those?

Thanks.

Answer 1

You are using IQueryable.Select method. So it is definitely translated into SQL.From the docs

The Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, TResult>>) method generates a MethodCallExpression that represents calling Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, TResult>> ) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

You can verify this by using a profiler.Or, try calling an unsupported method in the Select and you will get an exception that says linq to entities does not recognize the method X , that also verifies the Select is converted to SQL and not executed in the memory.

Answer 2

It will do both at the DB/ORM level. Results of your query fetched only when you enumerating your IQueriable eg calling ToList()