Subquery is a query inside a function that is a part of an outer query. If the outer query is a live view, and its base data changes so it needs to recompute the function, by default it just evaluates the function, which means creating a new view object for the subquery on every such recomputation. To improve performance by avoiding repeated creation and population of subviews, use the AttachView method to make the outer view aware of its subview. Then the outer view will cache and reuse subview objects it creates.

If there are more than one subviews attached to a single view, every subview must be supplied with a unique subview id by using the AttachView overload with subviewId parameter.

var productView =
    from p in products
    join s in sales on p.ProductID equals s.ProductID into productSales
    select new
    {
        p.Name,
        SalesByCountry = productSales.AttachView(v =>
            from s in v
            group s by s.Country into countrySales
            select new
            {
                Country = countrySales.Key,
                SaleCount = countrySales.LiveSum(s => s.Quantity).Value
            })
    };

This overload must be used if you have several subviews attached to the same view. The subqueryId can be any string as long as it is not repeated for two different subviews attached to the same view.

See the other overload for the explanation of the AttachView method.