Question

So I have a very simple structure:

• I have Orders that have a unique OrderNumber
• Orders have many OrderRows
• OrderRows have many RowExtras that have 2 fields, position (the sequence number of the RowExtra within the OrderRow) and Info, which is a string. More often than not, an OrderRow does not have more than one RowExtra.

(Don’t mind the silly structure for now, it’s just how it is).

So now I get a list of objects that have three properties:

• OrderNumber
• Position
• Info

What I want to do is simply 1) check if the RowExtra with the given OrderNumber/Position -pair exists in the database and if so, 2) update the Info-property.

I have tried a few different ways to accomplish this with very poor results at best. The solutions loop through the list of objects and issue a query such as

myContext.RowExtras.Where(x => x.Position == currentPosition &&
                          x.OrderRow.Order.OrderNumber == currentOrderNumber) 

or going from the other side

myContext.Orders.Where(x => x.OrderNumber == currentOrderNumber)
                .SelectMany(x => x.OrderRows)
                .SelectMany(x => x.RowExtras)
                .Where(x => x.Position == currentPosition)

and then check if the count equals to 1 and if so, update the property, otherwise proceed to next item.

I currently have roughly 4000 RowExtras in the database and need to update about half of them. These methods make the procedure take several minutes to complete, which is really not acceptable. What I don’t understand is why it takes such a long time, because the SQL-clause that returns the required RowExtra would be quite easy to write manually (with just 2 joins and 2 conditions in the where-part).

The best performance I managed to achieve was with a compiledquery looking like this

Func<MyContext, int, string, IQueryable<RowExtra>> query = 
CompiledQuery.Compile( 
  (MyContext ctx, int position, string orderNumber) => 
    from extra in ctx.RowExtras 
    where 
      extra.Position == position && 
      extra.OrderRow.Order == orderNumber 
    select extra);

and then invoking said query for each object in my list. But even this approach took way over a minute. So how do I actually get this thing to run within a reasonable timeframe?

Also, I’m sorry for the overly long explanation, but hopefully someone can help me!

Answer 1

Try to minimise the number of database calls. As a rule of thumb, each one will take roughly 10ms at least – even one that just returns a scalar.

So, in general, fetch all the data you will need in one go, modify it in code and then save it.

List<Order> orders = myContext.Orders
    .Include( "OrderRows.RowExtras" )
    .Where( ... select all the orders you want, not just one at a time ... )
    .ToList();

foreach ( Order order in orders )
{
    ... execute your logic on the in-memory model
}

myContext.SaveChanges();