mysql的order by rand方法實(shí)際運(yùn)用

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從一次查詢中隨機(jī)返回一條數(shù)據(jù)，一般使用mysql的order by rand() 方法來實(shí)現(xiàn)

例如: 從20萬用戶中隨機(jī)抽取1個(gè)用戶

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(jù)分析結(jié)果，運(yùn)行需要0.25秒，order by rand() 需要使用臨時(shí)表（Using temporary），需要使用文件排序（Using filesort），效率低下。

改進(jìn)方法

1.首先獲取查詢的總記錄條數(shù)total
2.在總記錄條數(shù)中隨機(jī)偏移N條（N=0~total-1)
3.使用limit N,1 獲取記錄
代碼如下：

<?php// 獲取總記錄數(shù)$sqlstr = 'select count(*) as recount from user';$query = mysql_query($sqlstr) or die(mysql_error());$stat = mysql_fetch_assoc($query);$total = $stat['recount'];// 隨機(jī)偏移$offset = mt_rand(0, $total-1);// 偏移后查詢$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)

本篇介紹了mysql order by rand() 效率優(yōu)化方法 ，更多相關(guān)內(nèi)容請關(guān)注億速云。

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