Query with GROUP BY not using Index

[Imran Akbar]

The following query is taking 3.8 seconds to run:
SELECT wafer_name FROM epi_iv GROUP BY wafer_name ORDER BY date DESC;

EXPLAIN SELECT wafer_name FROM epi_iv GROUP BY wafer_name ORDER BY date DESC\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: epi_iv
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 496613
Extra: Using temporary; Using filesort

SHOW CREATE TABLE's output is really long, but the relevant parts are:
`wafer_name` varchar(256) collate utf8_unicode_ci NOT NULL,
`date` datetime NOT NULL,
PRIMARY KEY (`iv_id`),
KEY `wafer_id` (`wafer_id`),
KEY `wafer_name` (`wafer_name`(255)),
KEY `wafer_name_date` (`wafer_name`(255),`date`)
) ENGINE=InnoDB AUTO_INCREMENT=514387 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

If I simplify the query by removing the ORDER BY, it still takes 3 seconds:
SELECT wafer_name FROM epi_iv GROUP BY wafer_name;
I know this query can run in ~.05 seconds when it uses the index because it does so on another table with about the same number of rows and the same composite index.

The cardinality of the wafer_name_date composite index is:
3463 for wafer_name and 126435 for the date.

Any ideas?

thanks