问查询视图时响应较慢-使用Linq to SQL

EN

SELECT     
poHeader.No_ AS PONumber, 
poHeader.[Buy-from Vendor No_] AS VendorNumber, 
poHeader.[Document Date] AS DocumentDate, 
vendor.Name AS VendorName, 
vendor.Contact AS VendorContact, 
vendor.[E-Mail] AS VendorEmail, 
vendor.Address AS VendorAddress, 
vendor.[Address 2] AS VendorAddress2, 
vendor.City AS VendorCity, 
vendor.County AS VendorCounty, 
vendor.[Post Code] AS VendorPostCode, 
vendor.[Phone No_] AS VendorPhone, 
vendor.[Fax No_] AS VendorFax, 
salesHeader.No_ AS SONumber, 
poHeader.[Order Date] AS OrderDate, 
salesHeader.[Crocus Comment] AS CrocusComment, 
salesHeader.GiftMessage, 
salesHeader.[Delivery Comment] AS DeliveryComment, 
salesHeader.[Shipment Date] AS DeliveryDate, 
COALESCE (salesHeader.[Ship-to Name], 
poHeader.[Ship-to Name]) AS DeliveryName, 
COALESCE (salesHeader.[Ship-to Address],
poHeader.[Ship-to Address]) AS DeliveryAddress, 
COALESCE (salesHeader.[Ship-to Address 2],
poHeader.[Ship-to Address 2]) AS DeliveryAddress2,
COALESCE (salesHeader.[Ship-to City], 
poHeader.[Ship-to City]) AS DeliveryCity, COALESCE (salesHeader.[Ship-to County], 
poHeader.[Ship-to County]) AS DeliveryCounty, 
COALESCE (salesHeader.[Ship-to Post Code], 
poHeader.[Ship-to Post Code]) AS DeliveryPostcode, 
salesHeader.DeliveryPhoneNo, poForEmailing.Processed, 
poForEmailing.Copied

FROM         
Navision4.dbo.[Crocus Live$Purch_ orders for e-mailing] AS poForEmailing 
LEFT OUTER JOIN
Navision4.dbo.[Crocus Live$Purchase Header] AS poHeader ON poForEmailing.No_ = poHeader.No_ 
INNER JOIN
Navision4.dbo.[Crocus Live$Vendor] AS vendor ON poHeader.[Buy-from Vendor No_] = vendor.No_ 
LEFT OUTER JOIN
Navision4.dbo.[Crocus Live$Sales Header] AS salesHeader ON salesHeader.No_ = dbo.fnGetSalesOrderNumber(poHeader.No_)

SELECT * 
FROM [NavisionMeta].[dbo].[PurchaseOrders]
WHERE Processed=0 AND Copied=0

            var purchaseOrdersNotProcessed = (from p in db.PurchaseOrders
                                              where p.Copied.Equals(0)
                                              && p.Processed.Equals(0)
                                              select p).ToList();

CREATE FUNCTION [dbo].[fnGetSalesOrderNumber](@PONumber varchar(20))
RETURNS varchar(20)
AS
BEGIN

RETURN (
SELECT 
    TOP 1 [Sales Order No_]
FROM 
    Navision4.dbo.[Crocus Live$Purchase Line]
WHERE 
    [Document No_] = @PONumber
)