Why running generic aggregation method is many times slower?

Why running generic aggregation method is many times slower?

I'm having performance problem with Entity Framework while using generic
methods for data aggregation. When querying max value of (indexed) Id
column of table with few hundred thousands of rows using generic methods
I'm getting huge performance drop. I'm using code-generated int keys
instead of sql identities, this code is used to get next new Id.
Here's sample illustration. MaxTyped uses Max on db.Posts (which is
DbSet<Post>), while MaxGeneric uses generic method max to do the same.
static int MaxTyped()
{
using (var db = new BloggingContext())
{
return db.Posts.Max(p => p.PostId);
}
}
static int MaxGeneric()
{
using (var db = new BloggingContext())
{
return max(db.Posts, p => p.PostId);
}
}
static int max<T>(DbSet<T> set, Func<T, int> func) where T : class
{
// intellisense says its IEnumerable.Max
return set.Max(func);
}
On my not so old home desktop with quite fast hdd, MaxTyped on 100k rows
runs in 0.5s, while MaxGeneric around 6.5s. This is one order of magnitude
slower.
On my office old testing server we're having times of few seconds vs
almost 10 minutes.
The only trace of the problem I've found is the difference in
Intellisense's output for Max method in both cases: in MaxTyped it
identifies method as IQueryable.Max, while in max used by MaxGeneric it
says its IEnumerable.Max, what might mean EF is doing Max on all loaded
entities and not in database. I tried casting set to IQueryable<T>, but
nothing changed. Why there's such a difference? How to fix it?