Merge a change from MySQL AB:

ChangeSet@1.2536.50.1  2007-08-02 12:45:56-07:00  igor@mysql.com

Fixed bug#28404.
This patch adds cost estimation for the queries with ORDER BY / GROUP BY
and LIMIT.
If there was a ref/range access to the table whose rows were required
to be ordered in the result set the optimizer always employed this access
though a scan by a different index that was compatible with the required
order could be cheaper to produce the first L rows of the result set.
Now for such queries the optimizer makes a choice between the cheapest
ref/range accesses not compatible with the given order and index scans
compatible with it.

innodb.result: Adjusted results for test cases affected fy the fix for
bug #28404.

mysql-test/innodb.result

@@ -948,7 +948,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	#	
 explain select * from t1 order by b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	b	4	NULL	#	
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	#	Using filesort
 explain select * from t1 order by c;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	#	Using filesort