《Mysql應(yīng)用MYSQL隨機(jī)抽取查詢 MySQL Order By Rand()效率問題》要點(diǎn)：
本文介紹了Mysql應(yīng)用MYSQL隨機(jī)抽取查詢 MySQL Order By Rand()效率問題，希望對(duì)您有用。如果有疑問，可以聯(lián)系我們。
要從tablename表中隨機(jī)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,后來我查了一下MYSQL的官方手冊(cè),里面針對(duì)RAND()的提示大概意思就是,在ORDER BY從句里面不能使用RAND()函數(shù),因?yàn)檫@樣會(huì)導(dǎo)致數(shù)據(jù)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(shí)現(xiàn)隨機(jī).
但是真正測(cè)試一下才發(fā)現(xiàn)這樣效率非常低.一個(gè)15萬余條的庫,查詢5條數(shù)據(jù),居然要8秒以上.查看官方手冊(cè),也說rand()放在ORDER BY 子句中會(huì)被執(zhí)行多次,自然效率及很低.
代碼如下:
You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY
would evaluate the column multiple times.
搜索Google,網(wǎng)上基本上都是查詢max(id) * rand()來隨機(jī)獲取數(shù)據(jù).
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(huì)產(chǎn)生連續(xù)的5條記錄.解決辦法只能是每次查詢一條,查詢5次.即便如此也值得,因?yàn)?5萬條的表,查詢只需要0.01秒不到.
下面的語句采用的是JOIN,mysql的論壇上有人使用
代碼如下:
SELECT * FROM `table` WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` ) ORDER BY id LIMIT 1;
再把語句完善一下,加上MIN(id)的判斷.我在最開始測(cè)試的時(shí)候,就是因?yàn)闆]有加上MIN(id)的判斷,結(jié)果有一半的時(shí)間總是查詢到表中的前面幾行.
完整查詢語句是：
代碼如下:
SELECT * FROM `table` 2 WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`))) 3 ORDER BY id LIMIT 1;
代碼如下:
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;
最后在php中對(duì)這兩個(gè)語句進(jìn)行分別查詢10次,
前者花費(fèi)時(shí)間 0.147433 秒
后者花費(fèi)時(shí)間 0.015130 秒
看來采用JOIN的語法比直接在WHERE中使用函數(shù)效率還要高很多.
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