In a mysql query, it is recommended that "GROUP BY" fields be indexed? Using 
EXPLAIN on one of my queries, I noticed no change between having indexed by 
GROUP BY field and not indexing it. Any thoughts would be appreciated.

In this example, should `history_field` be indexed...?

SELECT MAX(`timestamp`) AS `max_ts` 
FROM `history` 
WHERE `req_id` = 17 AND `history_record_id` = 35
GROUP BY `history_field`

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