Mysql query with multiple selects results in high CPU load - mysql

Question

I'm trying to do a link exchange script and run into a bit of trouble.
Each link can be visited by an IP address a number of x times (frequency in links table). Each visit costs a number of credits (spend limit given in limit in links table)
I've got the following tables:
CREATE TABLE IF NOT EXISTS `contor` (
`key` varchar(25) NOT NULL,
`uniqueHandler` varchar(30) DEFAULT NULL,
`uniqueLink` varchar(30) DEFAULT NULL,
`uniqueUser` varchar(30) DEFAULT NULL,
`owner` varchar(50) NOT NULL,
`ip` varchar(15) DEFAULT NULL,
`credits` float NOT NULL,
`tstamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`key`),
KEY `uniqueLink` (`uniqueLink`),
KEY `uniqueHandler` (`uniqueHandler`),
KEY `uniqueUser` (`uniqueUser`),
KEY `owner` (`owner`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE IF NOT EXISTS `links` (
`unique` varchar(30) NOT NULL DEFAULT '',
`url` varchar(1000) DEFAULT NULL,
`frequency` varchar(5) DEFAULT NULL,
`limit` float NOT NULL DEFAULT '0',
PRIMARY KEY (`unique`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
I've got the following query:
$link = MYSQL_QUERY("
SELECT *
FROM `links`
WHERE (SELECT count(key) FROM contor WHERE ip = '$ip' AND contor.uniqueLink = links.unique) <= `frequency`
AND (SELECT sum(credits) as cost FROM contor WHERE contor.uniqueLink = links.unique) <= `limit`")
There are 20 rows in the table links.
The problem is that whenever there are about 200k rows in the table contor the CPU load is huge.
After applying the solution provided by #Barmar:
Added composite index on (uniqueLink, ip) and droping all other indexes except PRIMARY, EXPLAIN gives me this:
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY l ALL NULL NULL NULL NULL 18
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 15
2 DERIVED pop_contor index NULL contor_IX1 141 NULL 206122

Answer 1

Try using a join rather than a correlated subquery.
SELECT l.*
FROM links AS l
LEFT JOIN (
SELECT uniqueLink, SUM(ip = '$ip') AS ip_visits, SUM(credits) AS total_credits
FROM contor
GROUP BY uniqueLink
) AS c
ON c.uniqueLink = l.unique AND ip_visits <= frequency AND total_credits <= limit
If this doesn't help, try adding an index on contor.ip.

Answer 2

The current query is of the form:
SELECT l.*
FROM `links` l
WHERE l.frequency >= ( SELECT COUNT(ck.key)
FROM contor ck
WHERE ck.uniqueLink = l.unique
AND ck.ip = '$ip'
)
AND l.limit >= ( SELECT SUM(sc.credits)
FROM contor sc
WHERE sc.uniqueLink = l.unique
)
Those correlated subqueries are going to each your lunch. And your lunchbox too.
I'd suggest testing an inline view that performs both of the aggregations from contor in one pass, and then join the result from that to the links table.
Something like this:
SELECT l.*
FROM ( SELECT c.uniqueLink
, SUM(c.ip = '$ip' AND c.key IS NOT NULL) AS count_key
, SUM(c.credits) AS sum_credits
FROM `contor` c
GROUP
BY c.uniqueLink
) d
JOIN `links` l
ON l.unique = d.uniqueLink
AND l.frequency >= d.count_key
AND l.limit >= d.sum_credits
For optimal performance of the aggregation inline view query, provide a covering index that MySQL can use to optimize the GROUP BY (avoiding a Using filesort operation)
CREATE INDEX `contor_IX1` ON `contor` (`uniqueLink`, `credits`, `ip`) ;
Adding that index renders the uniqueLink index redundant, so also...
DROP INDEX `uniqueLink` ON `contor` ;
EDIT
Since we have a guarantee that contor.key column is non-NULL (i.e. the NOT NULL constraint), this part of the query above is unneeded AND c.key IS NOT NULL, and can be removed. (I also removed the key column from the covering index definition above.)
SELECT l.*
FROM ( SELECT c.uniqueLink
, SUM(c.ip = '$ip') AS count_key
, SUM(c.credits) AS sum_credits
FROM `contor` c
GROUP
BY c.uniqueLink
) d
JOIN `links` l
ON l.unique = d.uniqueLink
AND l.frequency >= d.count_key
AND l.limit >= d.sum_credits