舊程式碼：
--receiptIds   id集合，每次查1000左右
var mappingList = new List<FinanceSettlementMapping>();
mappingList.AddRange(SettlementMappingRepository.Entities.Include(o => o.ReceiptsTo).Include(d => d.FinanceSettlement).Where(d => receiptIds.Contains(d.ReceiptIdFrom)).ToList());
mappingList.AddRange(SettlementMappingRepository.Entities.Include(o => o.ReceiptsTo).Include(d => d.FinanceSettlement).Where(d => receiptIds.Contains(d.ReceiptIdTo)).ToList());
--FinanceSettlementMapping -500w資料，9個欄位
--ReceiptsTo，外來鍵表，FinanceReceipts --800w資料，64個欄位
--FinanceSettlement  --200w資料，15個欄位
//FinanceSettlementMapping join FinanceSettlement join FinanceReceipts  表資料越大，連線越慢，可能會超時

新程式碼：外來鍵表FinanceReceipts資料單獨查詢
var mappingList = new List<FinanceSettlementMapping>();
var settlementmapFromList = SettlementMappingRepository.Entities.Include(d => d.FinanceSettlement).Where(d => receiptIds.Contains(d.ReceiptIdFrom)).ToList();
if (settlementmapFromList.Count > 0)
{
var toreceiptIds = settlementmapFromList.Select(o => o.ReceiptIdTo).ToArray();
var toreceipts = ReceiptsRepository.Entities.Where(o => toreceiptIds.Contains(o.ReceiptId)).ToList();//查詢資料到記憶體
foreach (var map in settlementmapFromList)//迴圈記憶體資料賦值
{
var tempreceipt = toreceipts.Where(o => o.ReceiptId == map.ReceiptIdTo).FirstOrDefault();
map.ReceiptsTo = tempreceipt;
}
}
mappingList.AddRange(settlementmapFromList);
var settlementmapToList = SettlementMappingRepository.Entities.Include(d => d.FinanceSettlement).Where(d => receiptIds.Contains(d.ReceiptIdTo)).ToList();
if (settlementmapToList.Count > 0)
{
var toreceiptIds = settlementmapToList.Select(o => o.ReceiptIdTo).ToArray();
var toreceipts = ReceiptsRepository.Entities.Where(o => toreceiptIds.Contains(o.ReceiptId)).ToList();//查詢資料到記憶體
foreach (var map in settlementmapToList)//迴圈記憶體資料賦值
{
var tempreceipt = toreceipts.Where(o => o.ReceiptId == map.ReceiptIdTo).FirstOrDefault();
map.ReceiptsTo = tempreceipt;
}
}

mappingList.AddRange(settlementmapToList);

//1000條資料，最差的線性查詢時間複雜度 （1+1000）*1000/2 ,本次查詢的是Id,是數字型別，如果系統用的是二叉樹或其他快速查詢方法，就更快，再者，查詢到之後，把toreceipts 值賦給map.ReceiptsTo，只是修改了map.ReceiptsTo的指標地址，沒有賦值操作，當然也是很快的，實測1000資料0.006s。

解決了大資料量表的Join查詢慢的問題，能加快程式總體執行速度，因為，我們網站應用 ，大部分時間是耗在資料庫操作上，所以能減少資料庫查詢時間，就能減少完成一個操作的整體執行時間。