mysql order by rand() 效率優化方法

轉載：https://blog.csdn.net/fdipzone/article/details/51541729 
 

從一次查詢中隨機返回一條資料，一般使用mysql的order by rand() 方法來實現

例如: 從20萬用戶中隨機抽取1個使用者

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)
 

根據分析結果，執行需要0.25秒，order by rand() 需要使用臨時表（Using temporary），需要使用檔案排序（Using filesort），效率低下。

改進方法
1.首先獲取查詢的總記錄條數total 
2.在總記錄條數中隨機偏移N條（N=0~total-1) 
3.使用limit N,1 獲取記錄 

程式碼如下：
 

<?php
// 獲取總記錄數
$sqlstr = 'select count(*) as recount from user';
$query = mysql_query($sqlstr) or die(mysql_error());
$stat = mysql_fetch_assoc($query);
$total = $stat['recount'];

// 隨機偏移
$offset = mt_rand(0, $total-1);

// 偏移後查詢
$sqlstr = 'select * from user limit '.$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)