mysql order by rand() 优化方法

mysql order by rand() 优化方法 适用于领取奖品等项目
<pre>
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)
</pre>
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
<pre>
<pre>
<?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);
?>
</pre>
<pre>
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)
</pre>
</pre>