quick selection of a random row from a large table in mysql

Question

What is a fast way to select a random row from a large mysql table?

I\'m working in php, but I\'m interested in any solution even if it\'s in another language.

Answer 1

I ran into the problem where my IDs were not sequential. What I came up with this.

SELECT * FROM products WHERE RAND()<=(5/(SELECT COUNT(*) FROM products)) LIMIT 1

The rows returned are approximately 5, but I limit it to 1.

If you want to add another WHERE clause it becomes a bit more interesting. Say you want to search for products on discount.

SELECT * FROM products WHERE RAND()<=(100/(SELECT COUNT(*) FROM pt_products)) AND discount<.2 LIMIT 1

What you have to do is make sure you are returning enough result which is why I have it set to 100. Having a WHERE discount<.2 clause in the subquery was 10x slower, so it's better to return more results and limit.