Using left join instead of union in EF with includeProperties - sql-server

Question

When I include multiple properties in EF it generates 'UNION ALL' SQL queries to include them. If I have 3 properties to include, it is almost 3 times slower to run it.
If I create the same query with 3 left joins it is performing much better...
Is it possible to use left join instead of union when linq is generating the query?
If it is not possible what is the reason to use union all, which seems slow?

Answer 1

First of all, you can perform left joins in LINQ to entities using DefaultIfEmpty.
Second of all, if your navigation properties are 1:N (and not 1:1 or 1:0..1), then using 3 joins will severely increase the output volume from the database and this will reduce the performance, in this case, running one query to retrieve the main entities, then 3 additional queries based on the IDs from the first query, ran in a single DB call using the .Future() extension, will result in better performance.
E.g.
var entities = context.Entities.AsQueryable().Where(...).ToList();
var ids = entities.select(e => e.Id).ToList();
var subEntities1Query = context.SubEntities1.AsQueryable().Where(se1 =>
ids.Contains(se1.ParentId)).Future();
var subEntities2Query = context.SubEntities2.AsQueryable().Where(se2 =>
ids.Contains(se2.ParentId)).Future();
var subEntities3Query = context.SubEntities3.AsQueryable().Where(se3 =>
ids.Contains(se3.ParentId)).Future();
var subEntities1 = subEntities1Query.ToList();
var subEntities2 = subEntities2Query.ToList();
var subEntities3 = subEntities3Query.ToList();
foreach (var entity in entities)
{
entity.SubEntities1 = subEntities1.Where(se1 =>
se1.ParentId = entity.Id).ToList();
entity.SubEntities2 = subEntities2.Where(se2 =>
se2.ParentId = entity.Id).ToList();
entity.SubEntities3 = subEntities3.Where(se3 =>
se3.ParentId = entity.Id).ToList();
}