order_by.result:

  result of the test case for FORCE INDEX on ORDER BY
order_by.test:
  test case for FORCE INDEX on ORDER BY
sql_select.cc:
  Changing behaviour that MySQL server takes FORCE INDEX clause into account when optimising ORDER BY clause

mysql-test/r/order_by.result

@@ -546,3 +546,11 @@ CREATE TABLE t1 (a INT, b INT);
 SET @id=0;
 UPDATE t1 SET a=0 ORDER BY (a=@id), b;
 DROP TABLE t1;
+create table t1(id int not null auto_increment primary key, t char(12));
+explain select id,t from t1 order by id;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	1000	Using filesort
+explain select id,t from t1 force index (primary) order by id;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	index	NULL	PRIMARY	4	NULL	1000	
+drop table t1;

mysql-test/t/order_by.test

@@ -352,4 +352,15 @@ CREATE TABLE t1 (a INT, b INT);
 SET @id=0;
 UPDATE t1 SET a=0 ORDER BY (a=@id), b;
 DROP TABLE t1;
-
+create table t1(id int not null auto_increment primary key, t char(12));
+disable_query_log;
+let $1 = 1000;
+while ($1)
+ {
+  eval insert into t1(t) values ('$1'); 
+  dec $1;
+ }
+enable_query_log;
+explain select id,t from t1 order by id;
+explain select id,t from t1 force index (primary) order by id;
+drop table t1;
\ No newline at end of file

sql/sql_select.cc

@@ -6021,7 +6021,7 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
     /* check if we can use a key to resolve the group */
     /* Tables using JT_NEXT are handled here */
     uint nr;
-    key_map keys=usable_keys;
+    key_map keys_to_use=~0,keys=usable_keys;
 
     /*
       If not used with LIMIT, only use keys if the whole query can be
@@ -6029,7 +6029,17 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
       retrieving all rows through an index.
     */
     if (select_limit >= table->file->records)
-      keys&= (table->used_keys | table->file->keys_to_use_for_scanning());
+      keys_to_use= (table->used_keys |table->file->keys_to_use_for_scanning());
+
+    /*
+      We are adding here also the index speified in FORCE INDEX clause, 
+      if any.
+      This is to allow users to use index in ORDER BY.
+    */
+ 
+    if (table->force_index) 
+      keys_to_use|= table->keys_in_use_for_query;
+    keys&= keys_to_use;
 
     for (nr=0; keys ; keys>>=1, nr++)
     {