Additional tweak to fix for bug #10543, to prevent a change in

behavior when extending fields that were fully part of a
multi-part key.

mysql-test/r/key.result

@@ -329,3 +329,28 @@ ERROR 42S21: Duplicate column name 'c1'
 alter table t1 add key (c1,c1,c2);
 ERROR 42S21: Duplicate column name 'c1'
 drop table t1;
+create table t1 (a varchar(10), b varchar(10), key(a(10),b(10)));
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` varchar(10) default NULL,
+  `b` varchar(10) default NULL,
+  KEY `a` (`a`,`b`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+alter table t1 modify b varchar(20);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` varchar(10) default NULL,
+  `b` varchar(20) default NULL,
+  KEY `a` (`a`,`b`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+alter table t1 modify a varchar(20);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` varchar(20) default NULL,
+  `b` varchar(20) default NULL,
+  KEY `a` (`a`,`b`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1;

mysql-test/t/key.test

@@ -324,3 +324,16 @@ alter table t1 add key (c1,c2,c1);
 --error 1060
 alter table t1 add key (c1,c1,c2);
 drop table t1;
+
+#
+# If we use a partial field for a key that is actually the length of the
+# field, and we extend the field, we end up with a key that includes the
+# whole new length of the field.
+#
+create table t1 (a varchar(10), b varchar(10), key(a(10),b(10)));
+show create table t1;
+alter table t1 modify b varchar(20);
+show create table t1;
+alter table t1 modify a varchar(20);
+show create table t1;
+drop table t1;

sql/sql_table.cc

@@ -3341,11 +3341,15 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
           previous type, or the field length is less than the key part
           length, unset the key part length.
 
+          We also unset the key part length if it is the same as the
+          old field's length, so the whole new field will be used.
+
           BLOBs may have cfield->length == 0, which is why we test it before
           checking whether cfield->length < key_part_length (in chars).
          */
         if (!Field::type_can_have_key_part(cfield->field->type()) ||
             !Field::type_can_have_key_part(cfield->sql_type) ||
+            cfield->field->field_length == key_part_length ||
 	    (cfield->length && (cfield->length < key_part_length /
                                 key_part->field->charset()->mbmaxlen)))
 	  key_part_length= 0;			// Use whole field