LINQ 延迟(或立即?)执行 [英] LINQ deferred (or immediate?) execution

问题描述

给定以下查询:

List<GetMultipleLookupListsOutput> data = await _masterListTranslationsRepository
.GetAll()    //<- it returns IQueriable
.GroupBy(q => q.ListLabelID)
.Select(q => q
   .OrderByDescending(w=>w.ISOLanguageCode == isoLanguageCode)
   .ThenByDescending(w=>w.ISOLanguageCode == "en-US"))
.Select(q => q.FirstOrDefault())   // DB call ?
.GroupBy(q=>q.ListLabels.Lists.ListName)
.Select(q => new GetMultipleLookupListsOutput
{
    ListName = q.Key,
    LookupLists = q
       .OrderByDescending(w => w.ISOLanguageCode == isoLanguageCode)
       .ThenByDescending(w => w.ISOLanguageCode == "en-US")
       .Select(w => new RegionalFeatureDto
       {
          Id = w.Id,
          Label = w.BaseValue
       })
       .ToList()   // DB call ?
})
.ToListAsync();

它将产生多少数据库调用?

How many database calls will it generate ?

GetAll() 方法返回 IQueryable，但第二次返回 FirstOrDefault() 和 ToList()第三个select语句会触发数据库调用?

GetAll() method returns IQueryable, but does FirstOrDefault() and ToList() in second and third select statements will trigger database call ?

任何帮助将不胜感激.

推荐答案

如果您担心生成多个调用，我会考虑使用 实体框架扩展

If you are concerned with generating multiple calls I would consider using EntityFramework Extensions

您可以通过将 .Future() 添加到查询末尾来一起批量查询

You can batch queries together by adding .Future() to the end of a query

例子:

db.BlogPosts.Where(x => x.Category.Any(y => y.Name.Contains("EntityFramework"))).Future();

因此，要回答您的问题，您可以将这些合并到对数据库的一次调用中.

So to answer your question you could combine these into one call to the database.

要检查 SQL/批处理，您还可以在查询之前包含以下内容:

To check the SQL/batching you can also include this before your query:

db.Database.Log = s => System.Diagnostics.Debug.WriteLine($"SQL: {s}");

日志将显示在您的输出窗口中.

and the log will be displayed in your output window.

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