EF Core header-detail 查询优化

Question

I have a header-detail relation in my SQL Server database. I have around 10k headers, each of them having 1-1k details. And the number of unique elements is about 1k.

Elements [id]
1
2
3

Headers [id]
1
2
3

Details [id, header_id, element_id]
1 1 1
2 1 2
3 1 3
4 2 1
5 3 1

It's very easy to query a list of headers with their details with such structure:

var searchHeaderIds = new List<int>{1,2,3};
var headers = context.Headers
                     .Where(h => searchHeaderIds.Contains(h.Id))
                     .Include(h => h.Details)
                     .ToList();

But what I want to query is a list of elements (1-200) where every element has a list of headers it belongs to (something like an inversion). I can write it in C# as below:

var searchElementIds = new List<int>{1,2,3};
var headers = context.Details
                     .Where(d => searchElementIds.Contains(d.element_id))
                     .GroupBy(d => d.element_id)
                     .Select(g => new { 
                                       id = g.Key, 
                                       header_ids = g.Select(x => x.header_id) })
                     .ToList();

But I wonder, what will be the fastest way to do it using the power of SQL/EF?

UPD: I'm ready to use extra data structures, preprocess the data in the database, or do anything else to improve performance.

Answer 1

what about:

var searchElementIds = new List<int>{1,2,3};
var headers = (
    from header in context.Headers
    join detail in context.Details on header.id equals detail.header_id
    where searchElementIds.Contains(detail.element_id)
    select header).Distinct();

If you want instances of the Element class:

var headers = 
    context.Details
        .Where(d => searchElementIds.Contains(d.element_id))
        .GroupBy(d => d.element_id)
        .Select(g => new Element 
                     { 
                         id = g.Key, 
                         header_ids = g.Select(x => x.header_id
                     })
        .ToList();

Don't cal ToList() in the middle of your query.

Answer 2

This is most optimal query in your case. It is closer to original post, but reduced number of retrieved fields for intermediate result:

var headers = context.Details
    .Where(d => searchElementIds.Contains(d.element_id))
    .Select(d => new { d.element_id, d.header_id })
    .ToList() // we need this, EF do not support retrieving grouping detals
    .GroupBy(d => d.element_id)
    .Select(g => new Element
    { 
        id = g.Key, 
        header_ids = g.Select(x => x.header_id).ToList()
    })
    .ToList();