我有以下结构的表:
-- 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;
以下查询的执行时间为9秒,显示30条记录。有什么问题?
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
分析结果
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
答案 0 :(得分:1)
COUNT DISTINCT
有时会导致MySql性能下降。试试这个:
select count(*) from (select distinct...
因为它有时会阻止MySql将整个中间结果写入磁盘。
以下是MySql错误信息: