如果你结合两个查询，我们是否需要首先在它们每个上运行ToList（）？

Question

我有一些LINQ to SQL查询，并且想要得到他们union的结果。我是否需要首先运行ToList()？或者我可以让他们作为IQueryable和工会他们和工会结束时只需拨打一个ToList()？这是否有任何性能改善？或者它本质上是一回事？

var query1 = (from a in this.Context.Blah select a); // .ToList()?? 
var query2 = (from b in this.Context.Yada select b); // .ToList()?? 
var query3 = (query1.Union(query2)).ToList(); 

Answer 1

方案1：

var query1 = (from a in this.Context.Blah select a); // you have a query 
var query2 = (from b in this.Context.Yada select b); // you have another query 
var query3 = (query1.Union(query2)); // you build a new query 
var result = query3.ToList(); // you execute a query and get result 

方案2：

//this builds a query and execute it and get result from database 
var query1 = (from a in this.Context.Blah select a).ToList(); // you execute a query 

//this builds a query and execute it and get result from database 
var query2 = (from b in this.Context.Yada select b).ToList(); // you execute another query 

//this builds a query and execute it but it does not hit database because you are working 
//with IEnumerable here not IQueryable 
var query3 = (query1.Union(query2)); // you build a new query 
var result = query3.ToList(); // you execute another query and get result 

情景2执行3查询，并有2个往返数据库，而第一个就去拿你想要什么一次往返。

构建查询并执行它有很大的区别。调用方法如ToList,ToArray,First,Single和...将执行您建立的查询，因为像Where,Union，Except和Except和Except这样的调用方法正在为您建立查询。

Answer 2

您应该让数据库联合数据集。它可能会比你做得更快，而且任何被删除的项目都不需要通过网络发送给你。