实体框架中的linq查询太慢

Question

I am using EF6 (model-first) that connects to Oracle 11G.

I have 3 tables A,B,C, and I defined the right FK to get the relationships:

1. B is a child of A.
2. C is a child of B.

Now, I'm running the following code (using eager loading) to get A with B and C:

var myQuery = from r in MyContext.A
                                 .Include(x => x.B.Select(y => y.C))
                                 .Where(a => a.sku == 2);
var res = myQuery.FirstOrDefault();

As you can see I use include and select to eagerly retrieve A with its B's and the B with its C's.

My problem is that it takes around 4 seconds to run the FirstOrDefault() method.

I have tested it on TOAD, and the generated SQL executed in less than one second.

The retrieved object (ie populated A), is not so big (A has 5 B's and each B has 2 C's, and each table has ~10 columns).

Is there a way to improve the performance?

Answer 1

I think that in your case .Include( a => aBSelect( y => yC ) ) will execute another separate SELECT for every row in your result-set, which is suboptimal, instead you should (generally) SELECT from the most descendant table first, then use Include to INNER JOIN any parent data that you need.

Try this query instead:

 var query = context.C
    .Include( c => c.B )
    .Include( c => c.B.A )
    .Where( c => c.B.A.sku == 2 );
return query.FirstOrDefault();

Also, check your indexes.

Answer 2

Eventually, to solve the slowness issue, I selected only the columns that I was interested in, and put it into a DTO.

var myQuery = from r in MyContext.A
                             .Include(x => x.B.Select(y => y.C))
                             .Where(a => a.sku == 2)
                              Select new MyDTO
                              {
                                 ...
                              }
var res = myQuery.FirstOrDefault();