Problems: 
  1. looking for a matching partition we miss the fact that the maximum 
     allowed value is in the PARTITION p LESS THAN MAXVALUE.
  2. one can insert maximum value if numeric maximum value is the last range.
     (should only work if LESS THAN MAXVALUE).
  3. one cannot have both numeric maximum value and MAXVALUE string as ranges 
    (the same value, but different meanings).

Fix: consider the maximum value as a supremum.