Tolist() 转换需要很长时间，我该如何改进？

Question

我有以下代码，其中使用 tolist 方法将数据从数据库转换为列表。我必须将整个数据转换为列表的原因是，我必须在使用 where 和 lambda 语句之后执行搜索操作，为此我们需要列表。

有什么替代方案吗？

// This takes less than 2 seconds to execute
var wdata = (from s in db.VIEW_ADDED_LOT
             select new LotModel
             {
                CREATION_DATE = s.CREATION_DATE,
                LOT_NO_SPL = s.LOT_NO_SPL,
                LOT_TYPE = s.LOT_TYPE,
                ITEM = s.ITEM,
                BUSINESS_UNIT = s.BUSINESS_UNIT,
                INSPECTOR = s.INSPECTOR,
                NCRNO = s.NCRNO,
                BUILDING_NO = s.BUILDING_NO,
                CELL = s.CELL,
                NCR_DT = s.NCR_DT,
                INVENTORY_ROUTER = s.INVENTORY_ROUTER,
                DOC_ISSUE = s.DOC_ISSUE,
                COMMENTS = s.COMMENTS,
                AGING = s.AGING,
                ARCHIVAL_DATE = s.ARCHIVAL_DATE,
                NCR_COMPLETION_STATUS = s.NCR_COMPLETION_STATUS,
                FLAG_LINK = s.FLAG_LINK,
                P_KEY = s.P_KEY
             });

// This takes around 1 minute to convert to list as there is 500 000 rows
var data = wdata.ToList();

// The reason why I am converting to list is that I have to perform n number of
// search on the basis of the filter chosen by user
if (NCR != null && NCR != "")
{
    data = data.Where(a => a.NCRNO == NCR).ToList();
}
if (LOT != null && LOT != "")
{
    data = data.Where(a => a.LOT_NO_SPL == LOT).ToList();
}

Answer 1

在您的示例中，wdata.ToList()调用评估查询并将整个查询加载wdata到内存中。您的初始分配wdata仅创建一个IQueryable对象，它实际上并不查询数据库。

为了避免性能下降，您应该将所有过滤器应用于然后在最后IQueryable调用，例如：ToList()

var data = wdata; // at this point its a queryable of your initial linq
if (NCR != null && NCR != "")
{
    data = data.Where(a => a.NCRNO == NCR); // appends one filter condition
}
if (LOT != null && LOT != "")
{
    data = data.Where(a => a.LOT_NO_SPL == LOT); // appends another filter condition
}
var finalResult = data.ToList(); 

这会将您的条件附加到IQueryable一旦您调用.ToList()，最终将得到解决，这意味着您不必将所有实体加载到内存中，因为过滤器将在数据库中进行评估。