Query with subquery and group by slower than expected

The whole query below runs incredibly slowly.

The subquery query [alias Stage_1] takes only 1.37 minutes returning 9514 records, however the whole query takes over 20 minutes, returning 2606 records.

I could use a #temp table to hold the subquery to improve the performance however I would prefer not to.

An overview of the query is that table WeeklySpace inner joins to Spaceblock_Name_to_PG table on SpaceblockName_SID, this cuts down the results in WeeklySpace and includes PG_Code with the results in WeeklySpace. WeeklySpace is then Full Outer Joined to Sales_PG_Wk across 3 fields. The where clause focuses the results, and may be changed. The results from the subquery are then sum'd. You cannot do the final sum'ing in the subquery due to the group by and sum over used.

I believe the issue is due to the subquery re calculation repeatedly during the group by in the final sum'ing. The field SpaceblockName_SID also appears to be involved in causing the issue as without it the run time with a group by in the subquery isn't affected.

I have read though loads of suggestion, trying them all to resolve the issue.

These include;

• Adding TOP 2147483647 with Order by to force intermediate materialization, both in the subquery and using a CTE.
• Adding a join after stage_1.
• Cast'ing SpaceblockName_SID from an int to a varchar and back again

The execution plan (cut in two parts, shown below the code) for both the subquery and the whole query appear similar. The cost is around the Full Outer Join (Hash Match), which I expected.

The query is running on T-SQL 2005.

Any help greatly appreciated!

select 
    Cost_centre
    , Fin_week
    , SpaceblockName_SID
    , sum(Propor_rep_SRV) as Total_SpaceblockName_SID_SRV
from
(
        select  
            coalesce(space_side.fin_week , sales_side.fin_week) as Fin_week 
           ,coalesce(space_side.cost_centre , sales_side.cost_Centre) as Cost_centre 
           ,space_side.SpaceblockName_SID
           ,case
                when  space_side.SpaceblockName_SID is null
                    then sales_side.SalesExVAT
                else  sum(space_side.TLM)
                                /nullif(sum (sum(space_side.TLM) ) over (partition by coalesce(space_side.fin_week , sales_side.fin_week)
                                                                                            , coalesce(space_side.cost_centre , sales_side.cost_Centre)  
                                                                                            , coalesce( Spaceblock_Name_to_PG.PG_Code, sales_side.PG_Code)) ,0)*sales_side.SalesExVAT 
            end as Propor_rep_SRV
        from
            WeeklySpace as space_side
        INNER JOIN
            Spaceblock_Name_to_PG 
                ON space_side.SpaceblockName_SID = Spaceblock_Name_to_PG.SpaceblockName_SID 
                    and Spaceblock_Name_to_PG.PG_Code < 10000
        full outer join
            sales_pg_wk as sales_side
                on  space_side.fin_week = sales_side.fin_week 
                    and space_side.Cost_Centre = sales_side.Cost_Centre 
                    and Spaceblock_Name_to_PG.PG_code = sales_side.pg_code
        where 
            coalesce(space_side.fin_week, sales_side.fin_week) between 201538 and 201550
                and
            coalesce(space_side.cost_centre, sales_side.cost_Centre) in (3, 2800)
        group by  
            coalesce(space_side.fin_week, sales_side.fin_week)  
           ,coalesce(space_side.cost_centre, sales_side.cost_Centre)  
           ,coalesce( Spaceblock_Name_to_PG.PG_Code, sales_side.PG_Code)  
           ,sales_side.SalesExVAT
           ,space_side.SpaceblockName_SID
) as stage_1
group by 
    Cost_centre
    , Fin_week
    , SpaceblockName_SID