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
一个伟大的职位处理几个情况,从简单,到差距,到不均匀与差距。
http://jan.kneschke.de/projects/mysql/order-by-rand/
对于大多数一般情况,你可以这样做:
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 1
这假设id的分布是相等的,并且id列表中可能存在间隙。有关更高级的示例,请参阅本文
其他回答
我得到了快速查询(大约0.5秒),cpu很慢,在一个400K寄存器的MySQL数据库中随机选择10行,非缓存2Gb大小。在MySQL中快速选择随机行
$time= microtime_float();
$sql='SELECT COUNT(*) FROM pages';
$rquery= BD_Ejecutar($sql);
list($num_records)=mysql_fetch_row($rquery);
mysql_free_result($rquery);
$sql="SELECT id FROM pages WHERE RAND()*$num_records<20
ORDER BY RAND() LIMIT 0,10";
$rquery= BD_Ejecutar($sql);
while(list($id)=mysql_fetch_row($rquery)){
if($id_in) $id_in.=",$id";
else $id_in="$id";
}
mysql_free_result($rquery);
$sql="SELECT id,url FROM pages WHERE id IN($id_in)";
$rquery= BD_Ejecutar($sql);
while(list($id,$url)=mysql_fetch_row($rquery)){
logger("$id, $url",1);
}
mysql_free_result($rquery);
$time= microtime_float()-$time;
logger("num_records=$num_records",1);
logger("$id_in",1);
logger("Time elapsed: <b>$time segundos</b>",1);
您可以轻松地使用带限制的随机偏移量
PREPARE stm from 'select * from table limit 10 offset ?';
SET @total = (select count(*) from table);
SET @_offset = FLOOR(RAND() * @total);
EXECUTE stm using @_offset;
您还可以像这样应用where子句
PREPARE stm from 'select * from table where available=true limit 10 offset ?';
SET @total = (select count(*) from table where available=true);
SET @_offset = FLOOR(RAND() * @total);
EXECUTE stm using @_offset;
在600,000行(700MB)表查询执行上的测试花费了大约0.016秒的硬盘驱动器时间。
EDIT:偏移量可能取接近表末尾的值,这将导致select语句返回更少的行(或者可能只有一行),为了避免这种情况,我们可以在声明偏移量后再次检查,如下所示
SET @rows_count = 10;
PREPARE stm from "select * from table where available=true limit ? offset ?";
SET @total = (select count(*) from table where available=true);
SET @_offset = FLOOR(RAND() * @total);
SET @_offset = (SELECT IF(@total-@_offset<@rows_count,@_offset-@rows_count,@_offset));
SET @_offset = (SELECT IF(@_offset<0,0,@_offset));
EXECUTE stm using @rows_count,@_offset;
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);
如果你想要一个随机记录(不管id之间是否有空隙):
PREPARE stmt FROM 'SELECT * FROM `table_name` LIMIT 1 OFFSET ?';
SET @count = (SELECT
FLOOR(RAND() * COUNT(*))
FROM `table_name`);
EXECUTE stmt USING @count;
来源:https://www.warpconduit.net/2011/03/23/selecting-a-random-record-using-mysql-benchmark-results/评论- 1266
这非常快,而且是100%随机的,即使你有间隙。
将SELECT Count(*)中可用的行数计算为rows FROM TABLE 选择10个不同的随机数a_1,a_2,…,a_10在0到x之间 SELECT * FROM TABLE LIMIT 1 offset a_i for i=1,…,10
我在Bill Karwin的《SQL反模式》一书中发现了这个破解方法。
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