MySQL Optimization Problem - mysql

Question

I'm having problems with a query optimization. The following query takes more than 30 seconds to get the expected result.
SELECT tbl_history.buffet_q_rating, tbl_history.cod_stock, tbl_history.bqqq_change_month, stocks.ticker, countries.country, stocks.company
FROM tbl_history
INNER JOIN stocks ON tbl_history.cod_stock = stocks.cod_stock
INNER JOIN exchange ON stocks.cod_exchange = exchange.cod_exchange
INNER JOIN countries ON exchange.cod_country = countries.cod_country
WHERE exchange.cod_country =125
AND DATE = '2011-07-25'
AND bqqq_change_month IS NOT NULL
AND buffet_q_rating IS NOT NULL
ORDER BY bqqq_change_month DESC
LIMIT 10
The tables are:
CREATE TABLE IF NOT EXISTS `tbl_history` (
`cod_stock` int(11) NOT NULL DEFAULT '0',
`date` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`price` decimal(11,3) DEFAULT NULL,
`buffet_q_rating` decimal(11,4) DEFAULT NULL,
`bqqq_change_day` decimal(11,2) DEFAULT NULL,
`bqqq_change_month` decimal(11,2) DEFAULT NULL,
(...)
PRIMARY KEY (`cod_stock`,`date`),
KEY `cod_stock` (`cod_stock`),
KEY `buf_rating` (`buffet_q_rating`),
KEY `data` (`date`),
KEY `bqqq_change_month` (`bqqq_change_month`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
CREATE TABLE IF NOT EXISTS `stocks` (
`cod_stock` int(11) NOT NULL AUTO_INCREMENT,
`cod_exchange` int(11) DEFAULT NULL,
PRIMARY KEY (`cod_stock`),
KEY `exchangestocks` (`cod_exchange`),
KEY `codstock` (`cod_stock`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=0 ;
CREATE TABLE IF NOT EXISTS `exchange` (
`cod_exchange` int(11) NOT NULL AUTO_INCREMENT,
`exchange` varchar(100) DEFAULT NULL,
`cod_country` int(11) DEFAULT NULL,
PRIMARY KEY (`cod_exchange`),
KEY `countriesexchange` (`cod_country`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=0 ;
CREATE TABLE IF NOT EXISTS `countries` (
`cod_country` int(11) NOT NULL AUTO_INCREMENT,
`country` varchar(100) DEFAULT NULL,
`initial_amount` double DEFAULT NULL,
PRIMARY KEY (`cod_country`),
KEY `codcountry` (`cod_country`),
KEY `country` (`country`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=0 ;
The first table have more than 20 million rows, the second have 40k and the others have just a few rows (maybe 100).
Them problem seems to be the "order by" but I have no idea how to optimize it.
I already tried some things searching on google/stackoverflow but I was unable to get good results
Can someone give me some advice?
EDIT:
Forgot the EXPLAIN result:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE countries const PRIMARY,codcountry PRIMARY 4 const 1 Using temporary; Using filesort
1 SIMPLE exchange ref PRIMARY,countriesexchange countriesexchange 5 const 15 Using where
1 SIMPLE stocks ref PRIMARY,exchangestocks,codstock exchangestocks 5 databaseName.exchange.cod_exchange 661 Using where
1 SIMPLE tbl_history eq_ref PRIMARY,cod_stock,buf_rating,data,bqqq_change_mont... PRIMARY 12 v.stocks.cod_stock,const 1 Using where
UPDATE
this is the new EXPLAIN I got:
id select_type table type possible_keys key key_len ref rows Extra |
1 SIMPLE tbl_history range monthstats monthstats 14 NULL 80053 Using where; Using index |
1 SIMPLE countries ref country country 4 const 1 Using index |
1 SIMPLE exchange ref PRIMARY,cod_country,countryexchange countryexchange 5 cons‌​t 5 Using where; Using index |
1 SIMPLE stocks ref info4stats info4stats 9 databaseName.exchange.cod_exchange,d‌​atabaseName.stock_... 1 Using where; Using index |

Answer 1

I would try to preemptively start with the Country records for 125 and work in reverse. By using a Straight_join will force the order of your query as entered...
I would also have an index on your Tbl_History table by the COD_Stock and DATE( date ). So the query will properly and efficiently match the join condition on the pre-qualified date portion of the date/time field.
SELECT STRAIGHT_JOIN
th.buffet_q_rating,
th.cod_stock,
th.bqqq_change_month,
stocks.ticker,
c.country,
s.company
FROM
Exchange e
join Countries c
on e.Cod_Country = c.Cod_Country
join Stocks s
on e.cod_exchange = s.cod_exchange
join tbl_history th
on s.cod_stock = th.cod_stock
AND th.`Date` = '2011-07-25'
AND th.bqqq_change_month IS NOT NULL
AND th.buffet_q_rating IS NOT NULL
WHERE
e.Cod_Country = 125
ORDER BY
th.bqqq_change_month DESC
LIMIT 10

Answer 2

If you want to limit the result, why do you do it after you join all the table?
Try to reduce the size of those big tables first (LIMIT or WHERE them) before joining them with other tables.
But you have to be sure that your original query and your modified query means the same.
Update (Sample) :
select
tbl_user.user_id,
tbl_group.group_name
from
tbl_grp_user
inner join
(
select
tbl_user.user_id,
tbl_user.user_name
from
tbl_user
limit
5
) as tbl_user
on
tbl_user.user_id = tbl_grp_user.user_id
inner join
(
select
group_id,
group_name
from
tbl_group
where
tbl_group.group_id > 5
) as tbl_group
on
tbl_group.group_id = tbl_grp_user.group_id
Hopefully, query above will give you a hint