EF SaveChanges优化？

Question

为什么在每个SaveChanges()之后处理DbContext ctx的速度更快？

第一个示例：

var ctx = new DomainContext(); 
foreach (var item in deals)
        {
            DealsOfReutersAddition newDealAddition =
                        new DealsOfReutersAddition
                        {
                            DealsOfReutersId = item.DealsOfReutersId,
                            DealDate = DateTime.Parse(item.DateConfirmed ?? "01 01 01", new CultureInfo("en-US")),
                        };
            ctx.DealsOfReutersAdditions.Add(newDealAddition);
            ctx.SaveChanges();
        }

ctx.Dispose();

第二个示例：

foreach (var item in deals)
        {
            DealsOfReutersAddition newDealAddition =
                        new DealsOfReutersAddition
                        {
                            DealsOfReutersId = item.DealsOfReutersId,
                            DealDate = DateTime.Parse(item.DateConfirmed ?? "01 01 01", new CultureInfo("en-US")),
                        };
            var ctx = new DomainContext(); 
            ctx.DealsOfReutersAdditions.Add(newDealAddition);
            ctx.SaveChanges();
            ctx.Dispose();
        }

在1000行上，第二个样本估计为8秒，而第一个样本为140秒。有没有办法清理ctx而不是重建？

Answer 1

按照@JensKloster在评论中所说的那样，你肯定不想在循环中每次都调用SaveChanges。但我实际上会抵制住把它放在循环之外的诱惑。随着上下文跟踪的对象数量越来越多，更多的内存将被耗尽，这将使存储变得越来越慢。

因此，我为您提供的解决方案是在循环中设置一个运行计数器，以确定何时执行保存：

int numberOfRecords = 0;
using(var ctx = new DomainContext())
{
  foreach (var item in deals)
          {
              DealsOfReutersAddition newDealAddition =
                          new DealsOfReutersAddition
                          {
                              DealsOfReutersId = item.DealsOfReutersId,
                              DealDate = DateTime.Parse(item.DateConfirmed ?? "01 01 01", new CultureInfo("en-US")),
                          };

              ctx.DealsOfReutersAdditions.Add(newDealAddition);

              numberOfRecords++;
              if(numberOfRecords % 500 == 0) //Saves after every 500 rows.
              {
                ctx.SaveChanges();
              }
           }
}