MySQL query slow in Where MONTH(datetime) - mysql

Question

I am trying to add index in datetime, but the result still same.
SELECT s.id, s.player,
COUNT(case when dg.winner = 1 AND dp.colour <= 5 then 1 when dg.winner = 2 AND dp.colour > 5 then 1 else null end) as totalwin,
COUNT(case when dg.winner = 2 AND dp.colour <= 5 then 1 when dg.winner = 1 AND dp.colour > 5 then 1 else null end) as totallose,
COUNT(dg.winner) as totalgames
FROM dotaplayers AS dp
LEFT JOIN gameplayers AS gp ON gp.gameid = dp.gameid and dp.colour = gp.colour
LEFT JOIN stats AS s ON s.player_lower = gp.name
LEFT JOIN dotagames AS dg ON dg.gameid = dp.gameid
LEFT JOIN games AS g ON g.id = dp.gameid
LEFT JOIN bans as b ON b.name=gp.name
WHERE MONTH(g.datetime) = 4
GROUP by gp.name
ORDER BY totalwin DESC LIMIT 0,10
Showing rows 0 - 9 (10 total, Query took 7.7552 seconds.)
I want order the most winner in 4th month (April). Then it shows id, username, totalwins, totallose, totaldraw, totalgames. The case in my query is the how to get that. The result is correct, but slow.

Answer 1

Assuming g.datetime is indexed, try this instead:
WHERE g.`datetime` BETWEEN 20150401 AND 20150430`
Using the MONTH function, or any other function, on the field data in the WHERE eliminates the benefits of any indexes you might have on those fields; this results in the query requiring a full scan of the values in the table.
Rearranging the order of JOINs will probably help as well:
SELECT s.id, s.player
, SUM(case
when dg.winner = 1 AND dp.colour <= 5 then 1
when dg.winner = 2 AND dp.colour > 5 then 1
else 0
end
) as totalwin
, SUM(case
when dg.winner = 2 AND dp.colour <= 5 then 1
when dg.winner = 1 AND dp.colour > 5 then 1
else 0
end
) as totallose
, COUNT(dg.winner) as totalgames -- Not, sure of the nature of dg.`winner`, a SUM might be more appropriate here as well.
FROM games AS g
INNER JOIN dotaplayers AS dp ON g.id = dp.gameid
LEFT JOIN gameplayers AS gp ON gp.gameid = dp.gameid and dp.colour = gp.colour
LEFT JOIN stats AS s ON s.player_lower = gp.name
LEFT JOIN dotagames AS dg ON dg.gameid = dp.gameid
LEFT JOIN bans as b ON b.name=gp.name
WHERE g.`datetime` BETWEEN 20150401000000 AND 20150430235959
GROUP by gp.name
ORDER BY totalwin DESC
LIMIT 0,10
;
Another thing to note: Depending on the relationship between tables, some of the intermediate joins may result in effectively multiplying the resulting totals; this can be resolved by doing the sums in subqueries and joining those instead.