Linq MVC5 MSQL select 包含大量列的表中的一些列，以便更好更快地查询

Question

For example a table that is containing like 30 columns

table1{col1,col2,col3,col4,col5,col6....,col30}

here we can do like

        List<table1> result = (from a in dbconn.table1.AsEnumerable()

                                        select new table1
                                        {
                                            col1= a.col1,
                                            col2= a.col2
                                        }).ToList();
        return result.ToList();

same as

 SELECT 
[Extent1].[col1] AS [col1], 
[Extent1].[col2] AS [col2],
[Extent1].[col3] AS [col3], 
....
....
....
[Extent1].[col30] AS [col30], 
FROM [dbo].[table1] AS [Extent1]

-> almost 2 minutes

but this is not correct because while doing this i am checking the result in SQL Server Profiler the result of this linq query is getting all the table table1 data then dumping to new table1 which is time consuming.

now this makes the query very slow because larg amount of data is there so in sql we can just

select col1,col2 from table1 

-> almost 2 seconds

how can we do it in linq with out getting the full table and make the linq same or near to SQL.

Answer 1

AsEnumerable() is effectively a cast to IEnumerable, which makes member resolution find members of Enumerable instead of Queryable. It's usually used when you want to force part of a query to run as SQL (or similar), and the remainder to run using LINQ to Objects.

(from a in dbconn.table1
     select new
     {
         col1= a.col1,
         col2= a.col2
     }).ToList();

Also, check indexes - will speed up more

Answer 2

The first rule to remember when working with LINQ to SQL is next:

Calling .ToList() , . ToArray() , .ToDictionary() and .AsIEnumerable() performs query materialization which means that your IQueriable will be transformed to SQL with all .Where() , .Select() , .Order() (etc.) predicates BEFORE it and will be executed.

In your example, you called .AsIEnumerable() directly on DbSet which means you will execute query without any predicates or limits and all data from this table from Database will be pushed to the memory and only after that your .Where() and .Select() will be applied, right in the memory