Проще, наверное, дать текущий код:

using System;
using System.Data;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using log4net;
using ExceptionHandler;

using Olympic.Misk.Range;
using Olympic.Orders.DeliveryStatus;
using Olympic.Orders.QtyDelivered;

namespace Olympic.Orders.UnfinishedOrders
{
    public static partial class TUnfinishedOrders
    {
        public partial class Table : Olympic.Shared.UnfinishedOrders.TUnfinishedOrders.Table
        {
            #region Logging

            private static readonly ILog logger = LogManager.GetLogger(typeof(Table).Name);

            #endregion Logging

            #region Local Data

            static protected DataTable empty;

            #endregion

            #region Constructor

            public Table() : base() { }
            protected internal Table(DataTable pDt) : base(pDt) { }

            #endregion

            #region Methods

            protected override CareyGlass.AbstractBO.Row GetNewRow() { return new Row(); }
            protected override CareyGlass.AbstractBO.Subset GetEmptySubset() { return new Subset(); }

            // here passed a Batch ranges
            public void Load(TRangeCollection pRanges)
            {
                //batchRanges = pRanges;
                Olympic.Sql.TGetCurrentOrders sqlt = new Olympic.Sql.TGetCurrentOrders();

                sqlt.Ranges = pRanges;
                //sqlt.Stations = new TKnownStations();

                try
                {
                    DataTable rawdata = DbExec.TExecFactory.LiProd.FillDataTable(sqlt);

                    if (empty == null) empty = rawdata.Clone();
                    rawdata.TableName = "DTBcdsToDo";

                    int rowsCount = rawdata.Rows.Count;

                    Table currentOrders = new Table(rawdata);
                    dt = GetUnfinishedOrders(currentOrders);
                }
                catch (Exception ex)
                {
                    TExceptionHandler.Error(ex);
                }
            } // check config!!!

            private DataTable GetUnfinishedOrders(Table pCurrentOrders)
            {
                Table unfinishedOrders = new Table(empty.Clone());

                TRange range = new TRange(pCurrentOrders.MinOrderNo, pCurrentOrders.MaxOrderNo);
                OrderRange = new TRangeCollection("DummyFieldName");
                OrderRange.Add(range);

                TDistinctOrders.Distinct distinctOrders = new TDistinctOrders.Distinct(pCurrentOrders);
                string orderList = distinctOrders.GetOrdersList();

                TDeliveryStatus.Table deliveryStatus = new TDeliveryStatus.Table();
                deliveryStatus.Load(OrderRange, orderList);

                TQtyDelivered.Table qtyDelivered = new TQtyDelivered.Table();
                qtyDelivered.Load(orderList);

                System.Diagnostics.Stopwatch stopwatch1 = System.Diagnostics.Stopwatch.StartNew();

                //var query1 = from current in pCurrent.Rows.Cast<DataRow>().Select(r => (Row)r)
                //            join delivery in deliveryStatus.Cast<TDeliveryStatus.Row>() on
 current.OrderNo equals delivery.OrderNo
                //            where delivery.DeliveryStatusS == TDeliveryStatus.NotDelivered
                //             // || delivery.DeliveryStatusS == TDeliveryStatus.PartiallyDelivered
                //            select current;

                var undeliveredAll = from current in pCurrentOrders.Cast<Row>().Select(r => r)
                                      join delivery in deliveryStatus.Cast<TDeliveryStatus.Row>() on current.OrderNo equals delivery.OrderNo
                                      where delivery.DeliveryStatusS == TDeliveryStatus.NotDelivered
                                      // || delivery.DeliveryStatusS == TDeliveryStatus.PartiallyDelivered
                                      orderby current.OrderNo, current.ItemNo
                                      select current;

                var partlyAll = from current in pCurrentOrders.Cast<Row>().Select(r => r)
                                 join delivery in deliveryStatus.Cast<TDeliveryStatus.Row>() on current.OrderNo equals delivery.OrderNo
                                 where delivery.DeliveryStatusS == TDeliveryStatus.PartiallyDelivered
                                 orderby current.OrderNo, current.ItemNo
                                 select current;

                var query3 = from undeliverd in partlyAll
                             join quantity in qtyDelivered.Cast<TQtyDelivered.Row>()
                                    on new { undeliverd.OrderNo, undeliverd.ItemNo }
                                    equals new { quantity.OrderNo, quantity.ItemNo }
                             orderby undeliverd.OrderNo, undeliverd.ItemNo
                             select new { undeliverd, quantity };

                int quantityCount = -1;
                int currentOrderNo = -1;
                int
 currentItemNo = -1;
                foreach (var pair in query3)
                {
                    if (pair.undeliverd.OrderNo != currentOrderNo
                     || pair.undeliverd.ItemNo != currentItemNo)
                    {
                        currentOrderNo = pair.undeliverd.OrderNo;
                        currentItemNo = pair.undeliverd.ItemNo;
                        quantityCount = 0;
                        //logger.InfoFormat("Order/item {0}/{1} required {2} not deliverd {3}, Restart counter", pair.undeliverd.OrderNo, pair.undeliverd.ItemNo, pair.quantity.QtyItem, pair.quantity.QtyCompNotDelivered);
                    }
                    if (++quantityCount <= pair.quantity.QtyCompNotDelivered)
                    {
                        pair.undeliverd.DeliveryStatus = TDeliveryStatus.PartiallyDelivered;
                        unfinishedOrders.ImportRow(pair.undeliverd);
                        //logger.InfoFormat("Order/item {0}/{1} required {2} not deliverd {3}, added", pair.undeliverd.OrderNo, pair.undeliverd.ItemNo, pair.quantity.QtyItem, pair.quantity.QtyCompNotDelivered);
                    }
                    else
                    {
                        //skip this pair
                        //logger.InfoFormat("Order/item {0}/{1} required {2} not deliverd {3}, skiped", pair.undeliverd.OrderNo, pair.undeliverd.ItemNo, pair.quantity.QtyItem, pair.quantity.QtyCompNotDelivered);
                    }
                }

                foreach (Row urow in undeliveredAll)
                {
                    urow.DeliveryStatus = TDeliveryStatus.NotDelivered;
                    unfinishedOrders.ImportRow(urow);
                }

                stopwatch1.Stop();

                System.Diagnostics.Debug.WriteLine("Selection for undelivered/partlidelivered took " + stopwatch1.Elapsed + "");
                System.Diagnostics.Debug.WriteLine("Rows selected: " + unfinishedOrders.dt.Rows.Count);

                //return unfinishedOrders.dt;
                return unfinishedOrders.dt.Select("", "orderNo,itemNo").CopyToDataTable();
            }


            #endregion

            #region Properties

            //public new Row FirstRow { get { return (Row)base.FirstRow; } }
            public TRangeCollection OrderRange { get; internal set; }

            #endregion

            #region Propeties Calculation

            public Decimal MinOrderNo { get { return Convert.ToDecimal(dt.Compute("MIN(orderNo)", "")); } }
            public Decimal MaxOrderNo { get { return Convert.ToDecimal(dt.Compute("MAX(orderNo)", "")); } }

            #endregion
        }
    }
}

Меня интересует выполнение фильтрации в цикле foreach (var pair in query3) ЛИНКом.

Чтобы упростилось до уровня foreach (Row urow in undeliveredAll)

Mожно и так оставить - оно достаточно быстрое - 0.5 сек на 20К записей, но все же интересно - можно ли слепить такой фильтр v LINQ.