On the other hand, you're likely to be taking a sample of rows that match a query. It's unlikely that the database already keeps track of the count of how many rows will match an arbitrary query, unless you set it up in advance. So you'd need to load the row ids of the matching rows, then sample from them.

A sufficiently smart database might understand "order by rand() limit n" and do reservoir sampling on row id's behind the scenes. I wouldn't count on it, though.

If you set it up in advance, you could also use reservoir sampling to avoid even storing all the data in the first place, while still saving a random sample.