我如何才能最好地编写一个查询,从总共600k中随机选择10行?
当前回答
我是这样做的:
select *
from table_with_600k_rows
where rand() < 10/600000
limit 10
我喜欢它,因为它不需要其他表,写起来很简单,执行起来非常快。
其他回答
我改进了@Riedsio的答案。这是我在一个有间隙的大型均匀分布表上能找到的最有效的查询(测试从一个有> 2.6B行的表中获得1000个随机行)。
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max := (SELECT MAX(id) FROM table)) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1)
让我来解释一下发生了什么。
(SELECT MAX(id) FROM table) 我在计算并保存最大值。对于非常大的表,每次需要一行时计算MAX(id)都会有轻微的开销 SELECT FLOOR(rand() * @max) + 1 as rand) 获取一个随机id SELECT id FROM table INNER JOIN(… 这就填补了空白。基本上,如果你在间隙中随机选择一个数字,它就会选择下一个id。假设间隙是均匀分布的,这应该不是问题。
进行联合可以帮助您将所有内容放入一个查询中,从而避免进行多个查询。它还可以节省计算MAX(id)的开销。根据您的应用程序,这可能非常重要,也可能无关紧要。
注意,这只获取id,并以随机顺序获取它们。如果你想做更高级的事情,我建议你这样做:
SELECT t.id, t.name -- etc, etc
FROM table t
INNER JOIN (
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max := (SELECT MAX(id) FROM table)) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1)
) x ON x.id = t.id
ORDER BY t.id
我需要一个查询从一个相当大的表中返回大量随机行。这是我想到的。首先获取最大记录id:
SELECT MAX(id) FROM table_name;
然后将该值代入:
SELECT * FROM table_name WHERE id > FLOOR(RAND() * max) LIMIT n;
Where max is the maximum record id in the table and n is the number of rows you want in your result set. The assumption is that there are no gaps in the record id's although I doubt it would affect the result if there were (haven't tried it though). I also created this stored procedure to be more generic; pass in the table name and number of rows to be returned. I'm running MySQL 5.5.38 on Windows 2008, 32GB, dual 3GHz E5450, and on a table with 17,361,264 rows it's fairly consistent at ~.03 sec / ~11 sec to return 1,000,000 rows. (times are from MySQL Workbench 6.1; you could also use CEIL instead of FLOOR in the 2nd select statement depending on your preference)
DELIMITER $$
USE [schema name] $$
DROP PROCEDURE IF EXISTS `random_rows` $$
CREATE PROCEDURE `random_rows`(IN tab_name VARCHAR(64), IN num_rows INT)
BEGIN
SET @t = CONCAT('SET @max=(SELECT MAX(id) FROM ',tab_name,')');
PREPARE stmt FROM @t;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET @t = CONCAT(
'SELECT * FROM ',
tab_name,
' WHERE id>FLOOR(RAND()*@max) LIMIT ',
num_rows);
PREPARE stmt FROM @t;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END
$$
then
CALL [schema name].random_rows([table name], n);
我认为这是一个简单但更快的方法,我在现场服务器上测试了它,与上面的几个答案相比,它更快。
SELECT * FROM `table_name` WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table_name` ) ORDER BY id LIMIT 30;
//对一个130行的表花费0.0014秒
SELECT * FROM `table_name` WHERE 1 ORDER BY RAND() LIMIT 30
//对130行的表花费0.0042秒
SELECT name
FROM random AS r1 JOIN
(SELECT CEIL(RAND() *
(SELECT MAX(id)
FROM random)) AS id)
AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 30
//对130行的表花费0.0040秒
如何从表中随机选择行:
从这里开始: 在MySQL中随机选择行
对“表扫描”的快速改进是使用索引来获取随机id。
SELECT *
FROM random, (
SELECT id AS sid
FROM random
ORDER BY RAND( )
LIMIT 10
) tmp
WHERE random.id = tmp.sid;
如果你只有一个读请求
将@redsio的答案与一个临时表结合起来(600K并不是很多):
DROP TEMPORARY TABLE IF EXISTS tmp_randorder;
CREATE TABLE tmp_randorder (id int(11) not null auto_increment primary key, data_id int(11));
INSERT INTO tmp_randorder (data_id) select id from datatable;
然后用一个@redsios的版本回答:
SELECT dt.*
FROM
(SELECT (RAND() *
(SELECT MAX(id)
FROM tmp_randorder)) AS id)
AS rnd
INNER JOIN tmp_randorder rndo on rndo.id between rnd.id - 10 and rnd.id + 10
INNER JOIN datatable AS dt on dt.id = rndo.data_id
ORDER BY abs(rndo.id - rnd.id)
LIMIT 1;
如果表比较大,可以先筛选第一部分:
INSERT INTO tmp_randorder (data_id) select id from datatable where rand() < 0.01;
如果你有很多读请求
Version: You could keep the table tmp_randorder persistent, call it datatable_idlist. Recreate that table in certain intervals (day, hour), since it also will get holes. If your table gets really big, you could also refill holes select l.data_id as whole from datatable_idlist l left join datatable dt on dt.id = l.data_id where dt.id is null; Version: Give your Dataset a random_sortorder column either directly in datatable or in a persistent extra table datatable_sortorder. Index that column. Generate a Random-Value in your Application (I'll call it $rand). select l.* from datatable l order by abs(random_sortorder - $rand) desc limit 1;
这个解决方案用最高和最低的random_sortorder来区分“边缘行”,所以在间隔中重新排列它们(一天一次)。
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