mySQL query is very slow after using DISTINCT and GROUP BY? - mysql

Question

I have tables with following structure:
-- Table structure for table `temp_app`
--
CREATE TABLE IF NOT EXISTS `temp_app` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`vid` int(5) NOT NULL,
`num` varchar(64) NOT NULL,
`timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
KEY `vid` (`vid`),
KEY `num` (`num`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=69509;
-- Table structure for table `inv_flags`
--
CREATE TABLE IF NOT EXISTS `inv_flags` (
`num` varchar(64) NOT NULL,
`vid` int(11) NOT NULL,
`f_special` tinyint(1) NOT NULL, /*0 or 1*/
`f_inserted` tinyint(1) NOT NULL, /*0 or 1*/
`f_notinserted` tinyint(1) NOT NULL, /*0 or 1*/
`userID` int(11) NOT NULL,
`timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
KEY `num` (`num`),
KEY `userID` (`userID`),
KEY `vid` (`vid`),
KEY `timestamp` (`timestamp`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Execution time of the following query is 9 seconds to display 30 records. What is wrong?
SELECT date_format(ifs.`timestamp`,'%y/%m/%d') as `date`
,count(DISTINCT ta.num) as inserted /*Unique nums*/
,SUM(ifs.f_notinserted) as not_inserted
,SUM(ifs.f_special) as special
,count(ta.num) as links /*All nums*/
from inventory_flags ifs
LEFT JOIN temp_app ta ON ta.num = ifs.num AND ta.vid = ifs.vid
WHERE ifs.userID = 3
GROUP BY date(ifs.`timestamp`) DESC LIMIT 30
EXPLAIN RESULT
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE ifs ref userID userID 4 const 12153 Using where
1 SIMPLE ta ref vid,num num 194 ifs.num 1

Answer 1

COUNT DISTINCT can sometimes cause rotten performance with MySql. Try this instead:
select count(*) from (select distinct...
as it can sometimes prevent MySql from writing the entire interim result to disk.
Here is the MySql bug info:
http://bugs.mysql.com/bug.php?id=21849