use tempdb;
go

create table dbo.t (id int identity primary key, g int, v float);

insert into dbo.t
select top (300000)
 rand(checksum(newid())) * 1000, rand(checksum(newid()))
from
 master.dbo.spt_values a cross join
 master.dbo.spt_values b;

create index IX_t__g__id on dbo.t (g, id) include (v);
go

declare @v float;

set statistics xml, time, io on;

select
 @v = sum(v) over (partition by g order by id)
from
 dbo.t
option
 (maxdop 1);

select
 @v = (select sum(v) from dbo.t where g = t1.g and id <= t1.id)
from
 dbo.t t1
option
 (maxdop 1);

select
 @v = sum(t2.v)
from
 dbo.t t1 join
 dbo.t t2 on t2.g = t1.g and t2.id <= t1.id
group by
 t1.g
option
 (maxdop 1);

set statistics xml, time, io off;
go

drop table dbo.t;
go

Table 'Worktable'. Scan count 301000, logical reads 1801405, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 't'. Scan count 1, logical reads 827, physical reads 0, read-ahead reads 28, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 3136 ms,  elapsed time = 3620 ms.


Table 't'. Scan count 300001, logical reads 1082463, physical reads 0, read-ahead reads 27, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 11466 ms,  elapsed time = 11530 ms.


Table 'Worktable'. Scan count 1000, logical reads 1505999, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 't'. Scan count 2, logical reads 1654, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 29921 ms,  elapsed time = 30154 ms.