One-to-many projection creates many subqueries

First of all - this tool practically saved my life when migrating from EF6 to EF Core, great job, everything works better than EF Core, instead of one thing

LinqToDB.Common.Configuration.Linq.AllowMultipleQuery = true;

var querySample = db.TicketNumbers.Where(p => p.Ticket.Name == "Lacna").Select(p => new
{
    Id = p.Id,
    TicketName = p.Ticket.Name,
    CustomFields = p.CustomFieldValues.Select(cf => new
    {
        Name = cf.CustomFieldDefinition.Name,
        Value = cf.Value
    }).ToList()
}).ToLinqToDB().ToList();

This guery yields back 300 results, from the logging it seems that Linq2Db core runs the subquery for each of the obtained item. [= 300 subqueries, 300 roundtrips to the DB]. For our scenario, for example, this creates an issue as our SQL Server is physically located elsewhere than the calling app creating a huge performance gap due to the latency, but I believe this might be an issue for other scenarios too (running 301 queries when in fact only 2 are needed)

From observing the logs EF Core handles this differently - running a single subquery on the very same condition as main “root” query, effectively creating only one more roundtrip to the DB, then linking the data based on their foreign-key declaration.

To sum it up, I have 3 questions:

• Is there anything I can do to change the one-to-many behavior?
• Are there any plans to change the subdata loading routine?
• If I wanted to hack a solution for our needs by changing the Linq2Db core, could you advise me where should I look?

[sdanyliv]

@brunolau, good news there. I had created first Eager Loading prototype using current sources and It looks very promising, later will point to PR which introduces this feature. A lot of work here but i hope in one or two weeks it will be done.

[sdanyliv]

I hope tomorrow.