MySQL select 10 random rows from 600K rows fast

Question

How can I best write a query that selects 10 rows randomly from a total of 600k?

Answer 1

Simple query that has excellent performance and works with gaps:

SELECT * FROM tbl AS t1 JOIN (SELECT id FROM tbl ORDER BY RAND() LIMIT 10) as t2 ON t1.id=t2.id

This query on a 200K table takes 0.08s and the normal version (SELECT * FROM tbl ORDER BY RAND() LIMIT 10) takes 0.35s on my machine.

This is fast because the sort phase only uses the indexed ID column. You can see this behaviour in the explain:

SELECT * FROM tbl ORDER BY RAND() LIMIT 10:

SELECT * FROM tbl AS t1 JOIN (SELECT id FROM tbl ORDER BY RAND() LIMIT 10) as t2 ON t1.id=t2.id

Weighted Version: https://stackoverflow.com/a/41577458/893432

Answer 2

This is how I do it:

select * 
from table_with_600k_rows
where rand() < 10/600000
limit 10

I like it because does not require other tables, it is simple to write, and it is very fast to execute.

Answer 3

How to select random rows from a table:

From here: Select random rows in MySQL

A quick improvement over "table scan" is to use the index to pick up random ids.

SELECT *
FROM random, (
        SELECT id AS sid
        FROM random
        ORDER BY RAND( )
        LIMIT 10
    ) tmp
WHERE random.id = tmp.sid;

Answer 4

I am getting fast queries (around 0.5 seconds) with a slow cpu, selecting 10 random rows in a 400K registers MySQL database non-cached 2Gb size. See here my code: Fast selection of random rows in MySQL

$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);

Answer 5

Use the below simple query to get random data from a table.

SELECT user_firstname ,
COUNT(DISTINCT usr_fk_id) cnt
FROM userdetails 
GROUP BY usr_fk_id 
ORDER BY cnt ASC  
LIMIT 10

Answer 6

I guess this is the best possible way..

SELECT id, id * RAND( ) AS random_no, first_name, last_name
FROM user
ORDER BY random_no