Question

我有一些LINQ to SQL个问题，想要获得union的结果。我是否需要先对每个人运行ToList()？或者我可以将它们留作IQueryable并将它们联合起来并在工会结束时只调用一个ToList()？这有没有任何性能提升？或者它基本上是一回事？

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

情景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

情景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

场景2执行3次查询并且有2次往返数据库，而第一次只是通过一次往返获取你想要的内容。

构建查询和执行查询之间存在很多差异。调用ToList，ToArray，First，Single和...之类的方法将执行您自调用Where，{{1}等方法后构建的查询}，Union和...正在为您构建查询。

Answer 2

您应该让数据库联合数据集。它可能比你能做得更快，并且任何被删除的项目都是不需要通过网络发送给你的项目。

如果你联合两个查询，我们是否需要先在每个查询上运行ToList（）？

2 个答案: