mysql order by rand() 效率優(yōu)化方法

例如: 從20萬用戶中隨機(jī)抽取1個用戶

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)

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