Don't client-side large objects [Possible 3.x regression]

This might be a 3.x regression. The code below was developed in 2.x and I’m 90% confident that it did what I want. We recently noticed a serious perf regression and observed the (new?) behavior described here.

Take a model that maps a DB blob into a .net byte[].

class File 
{
  public int Id { get; set; }
  public string Name { get; set; }
  public byte[] Data { get; set; }  // blob in DB
}

This query attempts to fetch the size of the first 20 records:

var q = from f in db.Files select new { f.Name, Length = f.Data.Length };
q.Take(20).ToList();

The SQL looks like this:

select Name, Data
from Files
where rownum <= 20

My gripe here is that linq2db fetches all of Data and chooses to evaluate Data.Length on the client-side.

Generally speaking, I love how linq2db evaluates as much as possible client-side. It gives access to all .net features that dbs don’t have and the results are consistent with other c# code (there’s always minor discrepancies in the implementation of “similar” functions).

In this specific instance, I don’t love it as much because Data is a heavy, costly object. Just imagine each blob is on average 5Mo. That query moves 100Mo over the network!! It’s especially nasty because it still “works”. We noticed the problem after significant perf. degradation.

I would suggest that byte[].Length is special-cased to always be server-side evaluated. I.e. above query should generate:

select Name, LENGTH(Data)
from Files
where rownum <= 20

Side note: I couldn’t find a built-in Sql.Length() that takes byte[], so working around this requires a bit of knowledge (I created my own length function).

Environment details

linq2db version: 3.1.4 Database Server: Oracle 12 Database Provider: Managed Operating system: Windows .NET Framework: Core 2.1

[jogibear9988]

@MaceWindu
It’s not so easy to fix…

Adding

		{ M(() => ((byte[])null!).Length ), N(() => L<byte[],int>((byte[] obj) => Sql.Length(obj)!.Value)) },

to Expressions does not work.

I then get this Exception: (Of cause, I added the Function before to SQL Class)

I’ll look if i can fix 😃

[MaceWindu]

I cannot find byte.Length mapping, so I’m pretty sure it is not regression. Anyway, we need to add Sql.Length function for byte[] (we have it only for Binary for some reason) and map byte[].Length/Binary.Length to it.