mysql order by rand() 效率优化方法

例如: 从20万用户中随机抽取1个用户

mysql> select * from user order by rand() limit 1;
+-------+------------+----------------------------------+----------+--------------+-----------+  id      phone        password                           salt       country_code   ip         
+-------+------------+----------------------------------+----------+--------------+-----------+  15160   6549721306   e4f302120c006880a247b652ad0e42f2   40343586   86             127.0.0.1  
+-------+------------+----------------------------------+----------+--------------+-----------+1 row in set (0.25 sec)mysql> explain select * from user order by rand() limit 1;
+----+-------------+-------+------+---------------+------+---------+------+--------+---------------------------------+  id   select_type   table   type   possible_keys   key    key_len   ref    rows     Extra                            
+----+-------------+-------+------+---------------+------+---------+------+--------+---------------------------------+   1   SIMPLE        user    ALL    NULL            NULL   NULL      NULL   200303   Using temporary; Using filesort  
+----+-------------+-------+------+---------------+------+---------+------+--------+---------------------------------+1 row in set (0.00 sec)

根据分析结果，运行需要0.25秒，order by rand() 需要使用临时表（Using temporary），需要使用文件排序（Using filesort），效率低下。

改进方法

1.首先获取查询的总记录条数total
2.在总记录条数中随机偏移N条（N=0~total-1)
3.使用limit N,1 获取记录
代码如下：

<?php// 获取总记录数$sqlstr = 'select count(*) as recount from user';$query = mysql_query($sqlstr) or die(mysql_error());$stat = mysql_fetch_assoc($query);$total = $stat['recount'];// 随机偏移$offset = mt_rand(0, $total-1);// 偏移后查询$sqlstr = 'select * from user limit '.$offset.',1';$query = mysql_query($sqlstr) or die(mysql_error());$result = mysql_fetch_assoc($query);
print_r($result);?>

分析：

mysql> select * from user limit 23541,1;
+-------+------------+----------------------------------+----------+--------------+-----------+  id      phone        password                           salt       country_code   ip         
+-------+------------+----------------------------------+----------+--------------+-----------+  23542   3740507464   c8bc1890de179538d8a49cc211859a46   93863419   86             127.0.0.1  
+-------+------------+----------------------------------+----------+--------------+-----------+1 row in set (0.01 sec)mysql> explain select * from user limit 23541,1;
+----+-------------+-------+------+---------------+------+---------+------+--------+-------+  id   select_type   table   type   possible_keys   key    key_len   ref    rows     Extra  
+----+-------------+-------+------+---------------+------+---------+------+--------+-------+   1   SIMPLE        user    ALL    NULL            NULL   NULL      NULL   200303   NULL   
+----+-------------+-------+------+---------------+------+---------+------+--------+-------+1 row in set (0.00 sec)

本篇介绍了mysql order by rand() 效率优化方法 ，更多相关内容请关注php中文网。

相关推荐：

解读php的PDO连接数据库的相关内容

讲解PHP面向对象，PHP继承相关代码

在PHP中使用魔术方法__CLASS__来获取类名的相关操作

以上就是mysql order by rand() 效率优化方法的详细内容，更多请关注php中文网其它相关文章！