Slow MySQL query

Hey I have a very slow MySQL query. I'm sure all I need to do is add the correct index but all the things I try don't work.

The query is:

SELECT DATE(DateTime) as 'SpeedDate', avg(LoadTime) as 'LoadTime'
FROM SpeedMonitor
GROUP BY Date(DateTime);

The Explain for the query is:

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  SpeedMonitor    ALL                 7259978 Using temporary; Using filesort

And the table structure is:

CREATE TABLE `SpeedMonitor` (
  `SMID` int(10) unsigned NOT NULL auto_increment,
  `DateTime` datetime NOT NULL,
  `LoadTime` double unsigned NOT NULL,
  PRIMARY KEY  (`SMID`)
) ENGINE=InnoDB AUTO_INCREMENT=7258294 DEFAULT CHARSET=latin1;

Any help would be greatly appreciated.

Solution

You're just asking for two columns in your query, so indexes could/should go there:

DateTime
LoadTime

Another way to speed your query up could be split DateTime field in two: date and time.
This way db can group directly on date field instead of calculating DATE(...).

EDITED:
If you prefer using a trigger, create a new column(DATE) and call it newdate, and try with this (I can't try it now to see if it's correct):

CREATE TRIGGER upd_check BEFORE INSERT ON SpeedMonitor
FOR EACH ROW
BEGIN
  SET NEW.newdate=DATE(NEW.DateTime);
END

EDITED AGAIN:
I've just created a db with the same table speedmonitor filled with about 900,000 records.
Then I run the query SELECT newdate,AVG(LoadTime) loadtime FROM speedmonitor GROUP BY newdate and it took about 100s!!
Removing index on newdate field (and clearing cache using RESET QUERY CACHE and FLUSH TABLES), the same query took 0.6s!!!
Just for comparison: query SELECT DATE(DateTime),AVG(LoadTime) loadtime FROM speedmonitor GROUP BY DATE(DateTime) took 0.9s.
So I suppose that the index on newdate is not good: remove it.
I'm going to add as many records as I can now and test two queries again.

FINAL EDIT:
Removing indexes on newdate and DateTime columns, having 8mln records on speedmonitor table, here are results:

selecting and grouping on newdate column: 7.5s
selecting and grouping on DATE(DateTime) field: 13.7s

I think it's a good speedup.
Time is taken executing query inside mysql command prompt.