How to find the rank of a sudoku without row reduction? Furthermore, is a sudoku always of full rank? When is it full rank and when is it not? I realized these questions are quite out there and there may not be any meaningful answer to this. 
 A: Using the MiniZinc constraint solver and Chuffed as solver back-end, I found the following rank $8$ sudoku:
9 1 6  2 7 4  5 3 8
2 7 5  8 6 3  4 9 1
8 4 3  5 1 9  6 2 7

6 2 9  1 3 7  8 5 4
4 5 1  6 2 8  9 7 3
7 3 8  4 9 5  2 1 6

3 8 7  9 5 6  1 4 2
1 9 4  7 8 2  3 6 5
5 6 2  3 4 1  7 8 9

Another rank 8 solution found using the MiniZinc G12 lazyfd solver back-end:
3 2 1  7 5 6  8 9 4
4 7 9  2 1 8  3 5 6
8 6 5  9 4 3  2 1 7

5 3 2  1 6 9  4 7 8
1 9 8  3 7 4  5 6 2
7 4 6  8 2 5  1 3 9

2 5 4  6 9 1  7 8 3
9 1 3  4 8 7  6 2 5
6 8 7  5 3 2  9 4 1

My model (based on this example):
int: s = 3;
int: n = s*s;
set of int: S = 1..s;
set of int: N = 1..n;

%  sudoku board is a n x n = s² x s² matrix:
array[N, N] of var N: puzzle;

%  column multipliers for full-rank determination
%  extra small range chosen to speed-up search
int: CMDIM = 1;
array[N] of var -CMDIM .. CMDIM: colMults;

include "alldifferent.mzn";

% All cells in a row, in a column, and in a subsquare are different.
constraint
    forall(i in N)( alldifferent(j in N)( puzzle[i,j] ))
    /\
    forall(j in N)( alldifferent(i in N)( puzzle[i,j] ))
    /\
    forall(i, j in S)
        ( alldifferent(p,q in S)( puzzle[s*(i-1)+p, s*(j-1)+q] ));

%  additional constraint to enforce non-full rank for puzzle matrix
%  Iff a linear combination of columns sums up to the null column,
%  we don't have full rank  
constraint forall(j in N) (
    0 = sum([colMults[i] * puzzle[j, i] | i in N])
  );  

%  exclude trivial solution with all-zero mutipliers
constraint exists([colMults[i] != 0 | i in N]);

solve satisfy;

function string: it(bool: cond, string: yes_s) =
  if cond then yes_s else "" endif;

function string: ite(bool: cond, string: yes_s, string :no_s) =
  if cond then yes_s else no_s endif;

output [ "sudoku:\n" ] ++
    [ show(puzzle[i,j]) ++
        if j = n then
            ite(i mod s = 0 /\ i < n, "\n\n", "\n")
        else
            ite(j mod s = 0, "  ", " ")
        endif
    | i,j in N ] ++
    ["\n rank {"] ++
    [ it(j = 1, "{") ++
      show(puzzle[i,j]) ++ 
      it(j < n, ",") ++
      it(j == n, "}") ++
      it((j == n) /\ (i < n), ",")
    | i,j in N ] ++
    ["}"] ++
    ["\nColumn Multipliers: "] ++
    [show(colMults[i]) ++ " " | i in N];

