【mysql】一条慢查询sql的的分析

这个是我在jobbole.com 上看到的

先给出数据表table结构

mysql> show create table tt \G
*************************** 1. row ***************************
       Table: tt
Create Table: CREATE TABLE `tt` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` char(16) NOT NULL DEFAULT '',
  `rule_id` int(10) unsigned NOT NULL DEFAULT '0',
  `status` tinyint(3) unsigned NOT NULL DEFAULT '1',
  PRIMARY KEY (`id`),
  KEY `ttx` (`name`,`rule_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1176504 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

目前数据库中数据大概有2000W条，之后可能还会不断增长，现在想要查询的是：从表中找出相同姓名，相同规则ID，并且status = 1对应的最后一条记录

写个程序随机加入数据

<?php

mysql_connect('10.0.0.234','root','123456');                                                                                                                                         
mysql_select_db('testdb');
mysql_query("set names utf8");
 
$strs ='';
$start = 'insert into tt values ';
for($i=0;$i<3000000;$i++){
 
    $k = range(a,z);
    shuffle($k);
    $str = implode('',$k);
    $num = mt_rand(8,16);
    $in = substr($str,0,$num);
 
    $strs .= " (NULL,'$in',$num,1),";
    if($i%10000 == 0){ 
        echo $i."\n";
        $sql = trim($start.$strs,',');
        mysql_query($sql);
        $strs ='';
    }   
}

这样300W数据就有了

 

目前的查询语句是这样的

SELECT * FROM tt WHERE id IN (
SELECT max(id) AS max_id FROM tt GROUP BY name, rule_id)
AND status = 1　

看一下查询计划

mysql> explain SELECT * FROM tt WHERE id IN ( SELECT max(id) AS max_id FROM tt GROUP BY name, rule_id) and status=1;
+----+--------------------+-------+-------+---------------+------+---------+------+---------+-------------+
| id | select_type        | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+--------------------+-------+-------+---------------+------+---------+------+---------+-------------+
|  1 | PRIMARY            | tt    | ALL   | NULL          | NULL | NULL    | NULL | 1176818 | Using where |
|  2 | DEPENDENT SUBQUERY | tt    | index | NULL          | ttx  | 52      | NULL |       1 | Using index |
+----+--------------------+-------+-------+---------------+------+---------+------+---------+-------------+

索引用上了，也没有filesort，这是不是就很快了，然后执行查询，就卡主了，卡主了，最后只能被Ctrl+C了

那么这个不行，就换一个，这里我想到了，子查询换成join看一看效果

SELECT a.id,b.name,b.rule_id  FROM (select max(id) as id from tt   group by name, rule_id) as a left join tt b on a.id = b.id

看一下查询计划

mysql> explain SELECT a.id,b.name,b.rule_id  FROM (select max(id) as id from tt   group by name, rule_id) as a left join tt b on a.id = b.id;
+----+-------------+------------+--------+---------------+---------+---------+------+---------+-------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows    | Extra       |
+----+-------------+------------+--------+---------------+---------+---------+------+---------+-------------+
|  1 | PRIMARY     | <derived2> | ALL    | NULL          | NULL    | NULL    | NULL | 1176503 |             |
|  1 | PRIMARY     | b          | eq_ref | PRIMARY       | PRIMARY | 4       | a.id |       1 |             |
|  2 | DERIVED     | tt         | index  | NULL          | ttx     | 52      | NULL | 1176818 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+------+---------+-------------+

执行结果  1.77579775 sec

mysql> show profiles;
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+
| Query_ID | Duration   | Query                                                                                                                         |
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+
|        6 | 1.77579775 | SELECT a.id,b.name,b.rule_id  FROM (select max(id) as id from tt   group by name, rule_id) as a left join tt b on a.id = b.id |
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+
7 rows in set (0.00 sec)

看一看执行 过程

mysql> show profile for query  6;     
+------------------------------+----------+
| Status                       | Duration |
+------------------------------+----------+
| Sending data                 | 0.000596 |
| Waiting for query cache lock | 0.000002 |
| Sending data                 | 0.000579 |
| Waiting for query cache lock | 0.000002 |
| Sending data                 | 0.000534 |
| Waiting for query cache lock | 0.000002 |
| Sending data                 | 1.101490 |
| end                          | 0.000010 |
| query end                    | 0.000004 |
| closing tables               | 0.000003 |
| removing tmp table           | 0.001369 |
| closing tables               | 0.000010 |
| freeing items                | 0.000024 |
| logging slow query           | 0.000002 |
| logging slow query           | 0.000035 |
| cleaning up                  | 0.000003 |
+------------------------------+----------+
100 rows in set (0.00 sec)

太多的查询缓存lock，关掉它，再看一下查询结果

mysql> show profile for query  10;   
+---------------------------+----------+
| Status                    | Duration |
+---------------------------+----------+
| starting                  | 0.000154 |
| checking permissions      | 0.000006 |
| checking permissions      | 0.000005 |
| Opening tables            | 0.000124 |
| System lock               | 0.000122 |
| optimizing                | 0.000007 |
| statistics                | 0.000035 |
| preparing                 | 0.000018 |
| executing                 | 0.000012 |
| Sorting result            | 0.000006 |
| Sending data              | 0.518897 |
| converting HEAP to MyISAM | 0.070147 |
| Sending data              | 0.067123 |
| init                      | 0.000033 |
| optimizing                | 0.000003 |
| statistics                | 0.000020 |
| preparing                 | 0.000009 |
| executing                 | 0.000001 |
| Sending data              | 1.193679 |
| end                       | 0.000011 |
| query end                 | 0.000010 |
| closing tables            | 0.000002 |
| removing tmp table        | 0.001491 |
| closing tables            | 0.000011 |
| freeing items             | 0.000020 |
| logging slow query        | 0.000002 |
| logging slow query        | 0.000050 |
| cleaning up               | 0.000003 |
+---------------------------+----------+

再次分析，我们发现 converting HEAP to MyISAM 这个很耗时

mysql> select @@max_heap_table_size/1024/1024;
+---------------------------------+
| @@max_heap_table_size/1024/1024 |
+---------------------------------+
|                     16.00000000 |
+---------------------------------+

mysql> set max_heap_table_size = 16777216*4;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@max_heap_table_size/1024/1024;
+---------------------------------+
| @@max_heap_table_size/1024/1024 |
+---------------------------------+
|                     64.00000000 |
+---------------------------------+

再看看执行结果  1.77579775 > 1.68962725 ，还是有效果的

mysql> show profiles;
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+
| Query_ID | Duration   | Query                                                                                                                         |
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+
|       17 | 1.68962725 | SELECT a.id,b.name,b.rule_id  FROM (select max(id) as id from tt   group by name, rule_id) as a left join tt b on a.id = b.id |
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+

分析

mysql> show profile for query  17;  
+----------------------+----------+
| Status               | Duration |
+----------------------+----------+
| starting             | 0.000120 |
| checking permissions | 0.000004 |
| checking permissions | 0.000003 |
| Opening tables       | 0.000106 |
| System lock          | 0.000113 |
| optimizing           | 0.000007 |
| statistics           | 0.000044 |
| preparing            | 0.000011 |
| executing            | 0.000006 |
| Sorting result       | 0.000002 |
| Sending data         | 0.567858 |
| init                 | 0.000032 |
| optimizing           | 0.000004 |
| statistics           | 0.000017 |
| preparing            | 0.000015 |
| executing            | 0.000002 |
| Sending data         | 1.120159 |
| end                  | 0.000011 |
| query end            | 0.000005 |
| closing tables       | 0.000002 |
| removing tmp table   | 0.001020 |
| closing tables       | 0.000011 |
| freeing items        | 0.000018 |
| logging slow query   | 0.000002 |
| logging slow query   | 0.000056 |
| cleaning up          | 0.000004 |
+----------------------+----------+

好看多了，耗时的地方都在 Sending data 上了，如果硬盘换成PCI-SSD 估计又能提高不少

 

还有没有其他方法呢，当然有，那么换一种写法

先看查询计划

mysql> explain select max(concat_ws(' ',lpad(id,5,' '),status)) as res, name, rule_id from tt group by name, rule_id ;
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------+
|  1 | SIMPLE      | tt    | index | NULL          | ttx  | 52      | NULL | 1176818 |       |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------+

　再看看执行结果

mysql> show profiles;
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+
| Query_ID | Duration   | Query                                                                                                                         |
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+
|       22 | 1.82505025 | select max(concat_ws(' ',lpad(id,5,' '),status)) as res, name, rule_id from tt group by name, rule_id                         |
+----------+------------+-------------------------------------------------------------------------------------------------------------------------------+

好像更慢了 1.82505025 > 1.68962725

分析一下

mysql> show profile for query  22;
+----------------------+----------+
| Status               | Duration |
+----------------------+----------+
| starting             | 0.000157 |
| checking permissions | 0.000007 |
| Opening tables       | 0.000019 |
| System lock          | 0.000020 |
| init                 | 0.000032 |
| optimizing           | 0.000005 |
| statistics           | 0.000016 |
| preparing            | 0.000012 |
| executing            | 0.000008 |
| Sorting result       | 0.000003 |
| Sending data         | 1.824677 |
| end                  | 0.000012 |
| query end            | 0.000005 |
| closing tables       | 0.000009 |
| freeing items        | 0.000016 |
| logging slow query   | 0.000002 |
| logging slow query   | 0.000049 |
| cleaning up          | 0.000004 |
+----------------------+----------+

时间基本上都花费在send data了

 

这里说一下，max + group by 完全就是一个坑，如果有多个字段要返回数据不能这样写

select  max(id) ,type ,name from table  where type=xx group by name

因为group by默认返回第一条记录

 

如果像下面那样写，肯定会遇到坑的

select max(id) as res, name, rule_id from tt group by name, rule_id ;

这样也可以，而且快多了，但是如果有其他字段怎么办

 

这样的语句基本上没有什么可以再优化了，只能换换其他方式了，比如：换SSD+raid10 ，分区/分表/分库+中间件了