That means, your field is a mixed value field. It contains not only numeric values but also alphanumeric values.
And thats why you get 3 different results:
Max(DiagnosisObjectID) ==> Maximum of the numeric values
Max(Fieldvalue('DiagnosisObjectID',RecNo())) ==> it's the same as max(DiagnosisObjectID), but I think you will see here a "?" as result, because in this case QlikView don't know how to interpret it (numeric or alphanumeric).
ORDER BY DiagnosisObjectID DESC ==> Maximum of ASCII values
and with MaxString(DiagnosisObjectID) you will get another differnt result.
Normally Max(DiagnosisObjectID) should be the fastest solution of all, but in the case of mixed values QlikView has at first to filter out the numeric values, before it can determine the max value of them and thats why it takes a few of minutes.
Maybe most of your values are filled with blanks on the right or left side.
