Not something to worry about mcuh. Just put everything in the clustered key and all is well.

create table tbl_statistics

(p_key int identity(1,1),

client_id int not null,

client_datetime datetime not null,

constraint CI_rowId_ClientId primary key clustered (p_key,client_id,client_datetime)

)

If you'd like to have datetime as nullable, than removing client_datetime from the index and it will not cause any changes . The table has just three short columns.