I have a fairly complex query that looks something like this:

create table Items(SomeOtherTableID int,SomeField int)
create table SomeOtherTable(Id int,GroupID int)
with cte1 as
(
	select 
		SomeOtherTableID,COUNT(*) SubItemCount 
	from 
		Items t 
	where 
		t.SomeField is not null 
	group by 
		SomeOtherTableID

),cte2 as
(
	select
		tc.SomeOtherTableID,ROW_NUMBER() over (partition by a.GroupID order by tc.SubItemCount desc) SubItemRank
	from 
		Items t
		inner join SomeOtherTable a on a.Id=t.SomeOtherTableID 
		inner join cte1 tc on tc.SomeOtherTableID=t.SomeOtherTableID
	where 
		t.SomeField is not null 

),cte3 as
(
	select 
		SomeOtherTableID 
	from 
		cte2 
	where 
		SubItemRank=1
)
select 
	* 
from 
	cte3 t1 
	inner join cte3 t2 on t1.SomeOtherTableID<t2.SomeOtherTableID 
option (maxdop 1)

The query is such that cte3 is filled with 6222 distinct results. In the final select, I am performing a cross join on cte3 with itself, (so that I can compare every value in the table with every other value in the table at a later point). Notice the final line :

option (maxdop 1)

Apparently, this switches off parallelism.

So, with 6222 results rows in cte3, I would expect (6222*6221)/2, or 19353531 results in the subsequent cross joining select, and with the final maxdop line in place, that is indeed the case.

However, when I remove the maxdop line, the number of results jumps to 19380454. I have 4 cores on my dev box.

WTF? Can anyone explain why this is? Do I need to reconsider previous queries that cross join in this way?