使用存在的EF查询会抛出内存不足异常

Question

Im trying to get a list of Shop_Orders where their order_num value does not exist in a table called WarhouseOrder . This table contains 500K records and there is an index on OrderNo . List<Shop_Order> contains approx 150 items, each of which has an order_num . When this code is executed, it seems very inefficient, slow and results in an outofmemory exception. Is there a better way to do this?

List<Shop_Order> new_orders = (from a in osource.order 
    where !ctx.WarehouseOrders.ToList()
    .Exists(o => o.OrderNo == a.order_num) select a).ToList();

Answer 1

WarehouseOrders.ToList() downloads all warehouse orders into memory. You can at least avoid that if you'll use Queryable.Any to check condition on database side:

!ctx.WarehouseOrders.Any(o => o.OrderNo == a.order_num)

But that will do database query for each shop order. I assume that you can get required shop orders with single database query. Eg you can do group join shop orders with warehouse orders and select only those which don't have matches. Something like:

from so in ctx.ShopOrders
join wo in ctx.WarehouseOrders on so.order_num equals wo.OrderNo into g
where !g.Any()
select so