Problem is not about intervals and doesn't actually cause 'full table scan'.
We have an optimization for DISTINCT when we have
'DISTINCT field_from_first_join_table' we don't need to read all the
rows from the JOIN-ed table if we found one conforming row.
It stopped working in 5.0 as we return NESTED_LOOP_OK if we came upon
that case in the evaluate_join_record() and that doesn't break the
recordreading loop in sub_select().

Fixed by returning NESTED_LOOP_NO_MORE_ROWS in this case.


mysql-test/r/select.result:
  Bug #32942 now() - interval '7200' second is NOT pre-calculated, causing "full table scan".
  
  test result
mysql-test/t/select.test:
  Bug #32942 now() - interval '7200' second is NOT pre-calculated, causing "full table scan"
  
  test case
sql/sql_select.cc:
  Bug #32942 now() - interval '7200' second NOT pre-calculated, causing "full table scan"
  
  return NESTED_LOOP_NO_MORE_ROWS when we don't need to read rows from
  this table anymore