Bug #30715: Assertion failed: item_field->field->real_maybe_null(),

  file .\opt_sum.cc, line
The optimizer pre-calculates the MIN/MAX values for queries like
 SELECT MIN(kp_k) WHERE kp_1 = const AND ... AND kp_k-1 = const
when there is a key over kp_1...kp_k
In doing so it was not checking correctly nullability and 
there was a superfluous assert(). 
Fixed by making sure that the field can be null before checking and
taking out the wrong assert().
.
Introduced a correct check for nullability 
The MIN(field) can return NULL when all the row values in the group
are NULL-able or if there were no rows.
Fixed the assertion to reflect the case when there are no rows.

mysql-test/r/func_group.result

@@ -1387,4 +1387,9 @@ SELECT 1 FROM t1 GROUP BY (SELECT SLEEP(0) FROM t1 ORDER BY AVG(DISTINCT a) );
 1
 1
 DROP TABLE t1;
+CREATE TABLE t1 (a int, b date NOT NULL, KEY k1 (a,b));
+SELECT MIN(b) FROM t1 WHERE a=1 AND b>'2007-08-01';
+MIN(b)
+NULL
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/func_group.test

@@ -873,5 +873,14 @@ SELECT 1 FROM t1 GROUP BY (SELECT SLEEP(0) FROM t1 ORDER BY AVG(DISTINCT a) );
 
 DROP TABLE t1;
 
+#
+# Bug #30715: Assertion failed: item_field->field->real_maybe_null(), file
+# .\opt_sum.cc, line
+#
+
+CREATE TABLE t1 (a int, b date NOT NULL, KEY k1 (a,b));
+SELECT MIN(b) FROM t1 WHERE a=1 AND b>'2007-08-01';
+DROP TABLE t1;
+
 ###
 --echo End of 5.0 tests

sql/opt_sum.cc

@@ -249,7 +249,8 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
                  Check if case 1 from above holds. If it does, we should read
                  the skipped tuple.
               */
-              if (ref.key_buff[prefix_len] == 1 && 
+              if (item_field->field->real_maybe_null() &&
+                  ref.key_buff[prefix_len] == 1 &&
                   /*
                      Last keypart (i.e. the argument to MIN) is set to NULL by
                      find_key_for_maxmin only if all other keyparts are bound
@@ -260,7 +261,6 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
                   (error == HA_ERR_KEY_NOT_FOUND ||
                    key_cmp_if_same(table, ref.key_buff, ref.key, prefix_len)))
               {
-                DBUG_ASSERT(item_field->field->real_maybe_null());
                 error= table->file->index_read(table->record[0], ref.key_buff,
                                                ref.key_length,
                                                HA_READ_KEY_EXACT);