Ef core search on child , order on parent - sql

Question

I have two model Parent and Child and I want to retrieve all Distinct parent that has unitid = 5 and order by family
public class Parent
{
public int Id { get; set; }
public string Family { get; set; }
public List<Child> Children { get; set; }
}
public class Child
{
public int Id { get; set; }
public int ParentId { get; set; }
public int UnitId { get; set; }
public Parent parent { get; set; }
}
parent table has 500000 record and child table has 2000000 record . I use SelectMany And Distinct but it take 8s to retrieve records
var parents= Context.Set<Parent>()
.SelectMany(x => x.Children.DefaultIfEmpty(),
(u, r) => new {
Parent = u,
Child = r })
.Where(x=>x.Child.UnitId == 5)
.Select(m => new Parent{
Id = m.Parent.Id,
Family= m.Parent.Family
})
.Distinct()
.OrderBy(x=>x.Family)
.Take(30).Skip(0).ToList();
Tranlate to :
SELECT [t].[Id], [t].[Family] FROM (
SELECT distinct [a].[Id], [a].[Family]
FROM [Parent] AS [a]
LEFT JOIN [Children] AS [r] ON [a].[Id] = [r].[ParentId]
WHERE ([r].[unitid] = 5) AS [t] ORDER BY [t].[family] OFFSET 0 ROWS FETCH next 30 ROWS ONLY
Or
var parents = Context.Set<Children>()
.Include(x=>x.Parent)
.Where(x=>x.UnitId ==5)
.Select(m => new Parent{
Id = m.ParentId ,
Family = m.Parent.Family})
.Distinct()
.OrderBy(x=>x.Family)
.Take(30).Skip(0).ToList();
Translatr to :
SELECT [t0].[Id],[t0].[Family] FROM ( SELECT DISTINCT [t].[Id], [t].[Family]
FROM [Children] AS [r]
INNER JOIN (
SELECT [a].[Id] , [a].[Family]
FROM [Parent] AS [a]
) AS [t] ON [r].[ParentId] = [t].[Id]
WHERE ([r].[UnitId] = 5) AS [t0] ORDER BY [t0].[Family] OFFSET 0 ROWS FETCH next 30 ROWS ONLY
1 . When use order by id it is very fast
2 . when use order by family and take,skip it is slow
3 . family is indexed
4 . Ef Core 3.1
Which solution is Correct or better ?

Answer 1

Check these two queries for performance:
var parents =
from p in Context.Set<Parent>()
where p.Children.Any(c => c.UnitId == 5)
orderby p.Family
select p;
parents = parents
.Take(30)
.Skip(0)
.ToList();
And another one:
var parentIds =
from c in Context.Set<Child>()
where c.UnitId == 5
select c.ParentId
var parents =
from p in Context.Set<Parent>()
join id in parentIds.Distinct() on p.Id equals id
orderby p.Family
select p;
parents = parents
.Take(30)
.Skip(0)
.ToList();