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

Here is a game changer that may be helpfully for many;

I have a table with 200k rows, with sequential id's, I needed to pick N random rows, so I opt to generate random values based in the biggest ID in the table, I created this script to find out which is the fastest operation:

logTime();
query("SELECT COUNT(id) FROM tbl");
logTime();
query("SELECT MAX(id) FROM tbl");
logTime();
query("SELECT id FROM tbl ORDER BY id DESC LIMIT 1");
logTime();

The results are:

• Count: 36.8418693542479 ms
• Max: 0.241041183472 ms
• Order: 0.216960906982 ms

Based in this results, order desc is the fastest operation to get the max id,
Here is my answer to the question:

SELECT GROUP_CONCAT(n SEPARATOR ',') g FROM (
    SELECT FLOOR(RAND() * (
        SELECT id FROM tbl ORDER BY id DESC LIMIT 1
    )) n FROM tbl LIMIT 10) a

...
SELECT * FROM tbl WHERE id IN ($result);

FYI: To get 10 random rows from a 200k table, it took me 1.78 ms (including all the operations in the php side)