Window Aggregate Logical Reads

drop table dbo.CS;
drop table dbo.T;

go

create table dbo.CS(CS bit not null, index IX_CS clustered columnstore);
create table dbo.T(I int not null, P int not null);
create index IX_T_P on dbo.T(P) include(I);

go

declare @NumRows int;
declare @NumPart int;

set @NumRows = 100000;
set @NumPart = 4

insert into dbo.T(I, P)
select top(@NumRows)
  row_number() over(order by(select null)),
  row_number() over(order by(select null)) % @NumPart
from sys.columns as C1,
     sys.columns as C2,
     sys.columns as C3

go

set statistics io on;

declare @I int;
select @I = min(I) over(partition by P)
from dbo.T
  left outer join dbo.CS on 0 = 1;

set statistics io off;

/*
Result on my machine
Microsoft SQL Server 2016 (SP1) (KB3182545) - 13.0.4001.0 (X64)

NumRows    NumPart   Logical reads in Worktable
100,000    4         2902
100,000    5         0
1,000,000  8         37535
1,000,000  9         0

*/