Merge

sql/key.cc:
  Auto merged
mysql-test/r/update.result:
  SCCS merged
mysql-test/t/update.test:
  SCCS merged

mysql-test/r/update.result

@@ -219,3 +219,10 @@ select * from t1;
 id	id_str
 1	test1
 drop table t1;
+create table t1 (a int, b char(255), key(a, b(20)));
+insert into t1 values (0, '1');
+update t1 set b = b + 1 where a = 0;
+select * from t1;
+a	b
+0	2
+drop table t1;

mysql-test/t/update.test

@@ -179,3 +179,13 @@ insert into t1 (id_str) values ("test");
 update t1 set id_str = concat(id_str, id) where id = last_insert_id();
 select * from t1;
 drop table t1;
+
+#
+# Bug #8942: a problem with update and partial key part
+#
+
+create table t1 (a int, b char(255), key(a, b(20)));
+insert into t1 values (0, '1');
+update t1 set b = b + 1 where a = 0;
+select * from t1;
+drop table t1;

sql/key.cc

@@ -293,7 +293,7 @@ bool check_if_key_used(TABLE *table, uint idx, List<Item> &fields)
     f.rewind();
     while ((field=(Item_field*) f++))
     {
-      if (key_part->field == field->field)
+      if (key_part->field->eq(field->field))
 	return 1;
     }
   }