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:
  Bug #30715: test case
mysql-test/t/func_group.test:
  Bug #30715: test case
sql/opt_sum.cc:
  Bug #30715: correct nullability check for MIN/MAX pre-calculation over index.

1 files changed, 5 insertions, 5 deletions

diff --git a/sql/opt_sum.cc b/sql/opt_sum.cc
index b9de54dbf5c..3fc62d05ae5 100644
--- a/sql/opt_sum.cc
+++ b/sql/opt_sum.cc
@@ -249,20 +249,20 @@ 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
                      to constants in a conjunction of equalities. Hence, we
                      can detect this by checking only if the last keypart is
                      NULL.
-                  */                     
+                  */
                   (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, 
+                                               ref.key_length,
                                                HA_READ_KEY_EXACT);
               }
             }