mysql使用rand随机查询记录效率测试一直以为mysql随机查询几条数据,就用 SELECT * FROM `table` ORDER BY RAND() LIMIT 5
就可以了。 查看官方手册,也说rand()放在ORDER BY 子句中会被执行多次,自然效率及很低。
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; 但是这样会产生连续的5条记录。解决办法只能是每次查询一条,查询5次。即便如此也值得,因为15万条的表,查询只需要0.01秒不到。 上面的语句采用的是JOIN,mysql的论坛上有人使用 SELECT *
FROM `table` WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` ) ORDER BY id LIMIT 1; 我测试了一下,需要0.5秒,速度也不错,但是跟上面的语句还是有很大差距。总觉有什么地方不正常。 于是我把语句改写了一下。 SELECT * FROM `table`
WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `table`))) ORDER BY id LIMIT 1; 这下,效率又提高了,查询时间只有0.01秒 最后,再把语句完善一下,加上MIN(id)的判断。我在最开始测试的时候,就是因为没有加上MIN(id)的判断,结果有一半的时间总是查询到表中的前面几行。 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; 最后在php中对这两个语句进行分别查询10次, |
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