result

Row subqueries producing no rows were not handled as UNKNOWN
values in row comparison expressions.

That was a result of the following two problems:

1. Item_singlerow_subselect did not mark the resulting row
value as NULL/UNKNOWN when no rows were produced.

2. Arg_comparator::compare_row() did not take into account that
a whole argument may be NULL rather than just individual scalar
values.

Before bug#34384 was fixed, the above problems were hidden
because an uninitialized (i.e. without any stored value) cached
object would appear as NULL for scalar values in a row subquery
returning an empty result. After the fix
Arg_comparator::compare_row() would try to evaluate
uninitialized cached objects.

Fixed by removing the aforementioned problems.


mysql-test/r/row.result:
  Added a test case for bug #54190.
mysql-test/r/subselect.result:
  Updated the result for a test relying on wrong behavior.
mysql-test/t/row.test:
  Added a test case for bug #54190.
sql/item_cmpfunc.cc:
  If either of the argument rows is NULL, return NULL as the
  result of comparison.
sql/item_subselect.cc:
  Adjust null_value for Item_singlerow_subselect depending on
  whether a row has been produced by the row subquery.