WHERE conditions 
 
check_group_min_max() checks if the loose index scan 
optimization is applicable for a given WHERE condition, that is 
if the MIN/MAX attribute participates only in range predicates 
comparing the corresponding field with constants. 
 
The problem was that it considered the whole predicate suitable 
for the loose index scan optimization as soon as it encountered 
a constant as a predicate argument. This is obviously wrong for 
cases when a constant is the first argument of a predicate 
which does not satisfy the above condition. 
 
Fixed check_group_min_max() so that all arguments of the input 
predicate are considered to decide if it passes the test, even 
though a constant has already been encountered.