Group selection
See my update of my previous post 
If I look at the linq to sql output, it looks like they're doing a groupjoin instead of a groupby, i.e.: per key a set of entities is fetched. THis is in fact a nested query hence the truckload of additional queries on their part.
I could 'hack' this in with a special case code path, where the groupby reference is converted into a nested query, as it seems (looking again at the docs) that is what group x by y means: group x under the key y in sets of x's.
mihies wrote:
Sorry for being late. Yes, it is useful to get grouping rows + actual rows that are part of each group. You can't do that with plain sql easily like that. I always shows this feature when doing LINQ to SQL presentations.
(then don't show the # of queries executed 
)
I'll give it a try later today. It's tricky I think, but the code is there so it should be possible to add this. The only drawback is to figure out when to build this special projection...
I think it's doable. It's a bit of work though... however the 'g' ends up as a SetReferenceExpression in the projection, and it is detectable if that's a groupby set. What then should be done is transform that setreferenceexpression into a nested query.
thus: ... select g;
becomes ... select new Grouping<keyType, IEnumerable<setType>> { g.Key, Values = gAsQueryWithoutGroupByClause};
Which then flows into the nested query engine just fine.
Almost there, the fake nested query is constructed, it needs a different expression to link it to the proper parent. Hopefully I get it finished today.
There's a workaround though.
See this example:
var q = from c in metaData.Customer
where c.Country!=null
group c by c.Country into g
select new
{
g.Key,
CustomersInCountry = (
from c2 in metaData.Customer
where g.Key == c2.Country
select c2)
};
Not pretty, but it works. 2 queries. Where the key is null it goes wrong in the projector as the resultset is of the wrong type, looking into that little issue (not related to this thread).
Got it working 
var q = from c in metaData.Customer group c by c.Country into g select new { g.Key, CustomersInCountry=g };
and
var q = from c in metaData.Customer where c.Country!=null group c by c.Country into g select g;
now work as expected. the first loads in CustomersPerCountry the ... customers per key (country) and the second produces a list of IGrouping<string, customer> instances, with 1 instance per key.
using 2 queries, 1 for the keys (where the group by is in) and 1 for the data to group. (take that, linq to sql )
Added to next build (which will be out in the next few days)
Hey Frans,
Nice work. No need to hurry with this update.
Build is now available