MDEV-40874 Wrong UPDATE change count for NULL fields on Aria

mysql-test/main/default.result

@@ -3490,3 +3490,36 @@ prepare stmt from "update t1 set f03 = ?";
 execute stmt using default;
 drop table t1;
 # End of 10.6 test
+#
+# MDEV-40874 Difference in affected row count on UPDATE ... SET col=DEFAULT
+#
+# A NULL row set back to its NULL default is unchanged and must report
+# Changed: 0, identically for every engine.  compare_record() used to
+# compare the undefined data bytes of a NULL field, so stale bytes made
+# the row look changed.
+create table t1 (c decimal(8,3)) engine=InnoDB;
+affected rows: 0
+insert into t1 values (null);
+affected rows: 1
+update t1 set c=default;
+affected rows: 0
+info: Rows matched: 1  Changed: 0  Warnings: 0
+drop table t1;
+affected rows: 0
+create table t1 (c decimal(8,3)) engine=Aria;
+affected rows: 0
+insert into t1 values (null);
+affected rows: 1
+update t1 set c=default;
+affected rows: 0
+info: Rows matched: 1  Changed: 0  Warnings: 0
+# A real change is still detected
+update t1 set c=1.5;
+affected rows: 1
+info: Rows matched: 1  Changed: 1  Warnings: 0
+# Setting DEFAULT (NULL) over a non-NULL value is a change
+update t1 set c=default;
+affected rows: 1
+info: Rows matched: 1  Changed: 1  Warnings: 0
+drop table t1;
+# End of 10.11 test

mysql-test/main/default.test

@@ -2193,3 +2193,29 @@ execute stmt using default;
 drop table t1;
 
 --echo # End of 10.6 test
+
+--echo #
+--echo # MDEV-40874 Difference in affected row count on UPDATE ... SET col=DEFAULT
+--echo #
+--echo # A NULL row set back to its NULL default is unchanged and must report
+--echo # Changed: 0, identically for every engine.  compare_record() used to
+--echo # compare the undefined data bytes of a NULL field, so stale bytes made
+--echo # the row look changed.
+--source include/have_innodb.inc
+--source include/have_maria.inc
+--enable_info
+create table t1 (c decimal(8,3)) engine=InnoDB;
+insert into t1 values (null);
+update t1 set c=default;
+drop table t1;
+create table t1 (c decimal(8,3)) engine=Aria;
+insert into t1 values (null);
+update t1 set c=default;
+--echo # A real change is still detected
+update t1 set c=1.5;
+--echo # Setting DEFAULT (NULL) over a non-NULL value is a change
+update t1 set c=default;
+--disable_info
+drop table t1;
+
+--echo # End of 10.11 test

sql/sql_update.cc

@@ -96,6 +96,8 @@ bool compare_record(const TABLE *table)
           if (((table->record[0][null_byte_index]) & field->null_bit) !=
               ((table->record[1][null_byte_index]) & field->null_bit))
             return TRUE;
+          if (field->is_null())
+            continue;
         }
         if (field->cmp_binary_offset(table->s->rec_buff_length))
           return TRUE;
@@ -121,6 +123,7 @@ bool compare_record(const TABLE *table)
   {
     Field *field= *ptr;
     if (field->has_explicit_value() && !field->vcol_info &&
+	!field->is_null() &&
 	field->cmp_binary_offset(table->s->rec_buff_length))
       return TRUE;
   }

sql/table.cc

@@ -3467,7 +3467,8 @@ int TABLE_SHARE::init_from_binary_frm_image(THD *thd, bool write,
   share->last_null_bit_pos= null_bit_pos;
   share->null_bytes_for_compare= null_bits_are_used ? share->null_bytes : 0;
   share->can_cmp_whole_record= (share->blob_fields == 0 &&
-                                share->varchar_fields == 0);
+                                share->varchar_fields == 0 &&
+                                !null_bits_are_used);
 
   data_start= share->default_values;
   data_end= data_start + share->reclength;