子查询和集团在LINQ

Question

这种查询是写在我们的系统中，前一段时间，但此查询的表现越来越差，在数据略有增加。我的调查显示（CodeCount）查询触发另一个子查询导致执行大量延迟。我需要优化这个Linq查询。任何帮助将不胜感激

from batch in Context.VoucherCodeBatch.ToList() 
        join type in Context.VoucherCodeType on batch.VoucherTypeId equals type.VoucherTypeId 
        join voucher in Context.Voucher on batch.VoucherCodeBatchId equals voucher.VoucherCodeBatchId 

        where batchIds.Contains(batch.BatchCode) 
        group new 
        { 
         batch.BatchCode, 
         batch.CreationDate, 
         type.VoucherTypeName, 
         voucher.AllowedCount, 
         voucher.ValidFrom, 
         voucher.ValidTo, 
         batch.VoucherCodeBatchId, 
         voucher.VoucherCode 
        } 
         by new { batch.BatchCode } 
         into uniquebatch 
         select new Batch 
         { 
          BatchCode = uniquebatch.FirstOrDefault().BatchCode, 
          CreationDate = uniquebatch.FirstOrDefault().CreationDate, 
          TimesAllowed = uniquebatch.FirstOrDefault().AllowedCount, 
          ValidFrom = uniquebatch.FirstOrDefault().ValidFrom, 
          CodeCount = ((from c in Context.Voucher.ToList() 
              where 
               c.VoucherCodeBatchId == 
               uniquebatch.FirstOrDefault().VoucherCodeBatchId 
              select c).Count()), 
          ValidTo = uniquebatch.FirstOrDefault().ValidTo, 
          CodeType = uniquebatch.FirstOrDefault().VoucherTypeName, 
          VoucherCodeBatchId = uniquebatch.FirstOrDefault().VoucherCodeBatchId 
         }); 

Answer 1

第一个大问题是ToList()权的对象集<前>（表收集EF）。

决不做，ToList()力EF把所有的数据存储过程之前查询。 （就像@Daniel Hilgarth评论）。

其他细节是符合使用的FirstOrDefault()前面获取属性，如：在这种情况下

BatchCode = uniquebatch.FirstOrDefault().BatchCode, 

使用First()代替FirstOrDefault。像：

BatchCode = uniquebatch.First().BatchCode, 

您的查询将是这样的：

from batch in Context.VoucherCodeBatch/*.ToList()*/ 
join type in Context.VoucherCodeType on batch.VoucherTypeId equals type.VoucherTypeId 
join voucher in Context.Voucher on batch.VoucherCodeBatchId equals voucher.VoucherCodeBatchId 
where batchIds.Contains(batch.BatchCode) 
group new 
    { 
     batch.BatchCode, 
     batch.CreationDate, 
     type.VoucherTypeName, 
     voucher.AllowedCount, 
     voucher.ValidFrom, 
     voucher.ValidTo, 
     batch.VoucherCodeBatchId, 
     voucher.VoucherCode 
    } 
by new { batch.BatchCode } 
into uniquebatch 
select (delegate 
    { 
     // If you put a operation in a query that operation will be 
     // processed all times. Bacause that i removed this line from 
     // the where statement. 
     var vcBatchId = uniquebatch.First().VoucherCodeBatchId; 

     return new Batch 
      { 
       BatchCode = uniquebatch.First().BatchCode, 
       CreationDate = uniquebatch.First().CreationDate, 
       TimesAllowed = uniquebatch.First().AllowedCount, 
       ValidFrom = uniquebatch.First().ValidFrom, 
       CodeCount = ((
        from c in Context.Voucher/*.ToList()*/ 
        where c.VoucherCodeBatchId == vcBatchId 
        select c).Count()), 
       ValidTo = uniquebatch.First().ValidTo, 
       CodeType = uniquebatch.First().VoucherTypeName, 
       VoucherCodeBatchId = uniquebatch.First().VoucherCodeBatchId 
      } 
    }); 

如果这种改进是不够好，你需要改变这个查询SQL。但我相信这种改善会更好。