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

You can easily use a random offset with a limit

PREPARE stm from 'select * from table limit 10 offset ?';
SET @total = (select count(*) from table);
SET @_offset = FLOOR(RAND() * @total);
EXECUTE stm using @_offset;

You can also apply a where clause like so

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;

Tested on 600,000 rows (700MB) table query execution took ~0.016sec HDD drive.

EDIT: The offset might take a value close to the end of the table, which will result in the select statement returning less rows (or maybe only 1 row), to avoid this we can check the offset again after declaring it, like so

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;