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 | 37405074 | c8bc10de179538d8a49cc211859a46 | 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)

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