MySQL Order By Rand()效率分析

最近由于需要大概研究了一下MYSQL的随機抽取實作方法。舉個例子，要從tablename表中随機提取一條記錄，大家一般的寫法就是：SELECT * FROM tablename ORDER BY RAND() LIMIT 1。 

但是，後來我查了一下MYSQL的官方手冊，裡面針對RAND()的提示大概意思就是，在ORDER BY從句裡面不能使用RAND()函數，因為這樣會導緻資料列被多次掃描。但是在MYSQL 3.23版本中，仍然可以通過ORDER BY RAND()來實作随機。 

但是真正測試一下才發現這樣效率非常低。一個15萬餘條的庫，查詢5條資料，居然要8秒以上。檢視官方手冊，也說rand()放在ORDER BY 子句中會被執行多次，自然效率及很低。 

You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. 

搜尋Google，網上基本上都是查詢max(id) * rand()來随機擷取資料。 

複制代碼 代碼如下:

SELECT * 

 FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `table`)) AS id) AS t2 

 WHERE t1.id >= t2.id 

 ORDER BY t1.id ASC LIMIT 5;      

但是這樣會産生連續的5條記錄。解決辦法隻能是每次查詢一條，查詢5次。即便如此也值得，因為15萬條的表，查詢隻需要0.01秒不到。 

下面的語句采用的是JOIN，mysql的論壇上有人使用 

複制代碼 代碼如下:

SELECT * 

 FROM `table` 

 WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` ) 

 ORDER BY id LIMIT 1;      

我測試了一下，需要0.5秒，速度也不錯，但是跟上面的語句還是有很大差距。總覺有什麼地方不正常。 

于是我把語句改寫了一下。 

SELECT * FROM `table` 

 WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `table`))) 

 ORDER BY id LIMIT 1;      

SELECT * FROM `table` 

 WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`))) 

 ORDER BY id LIMIT 1;      

SELECT * 

 FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`))+(SELECT MIN(id) FROM `table`)) AS id) AS t2 

 WHERE t1.id >= t2.id 

 ORDER BY t1.id LIMIT 1;