Note that `FROM table TABLESAMPLE SYSTEM (n/N)` is a safe substituion for `ORDER BY random() LIMIT n` only when you don't need a sample that's guaranteed to be

Note that `FROM table TABLESAMPLE SYSTEM (n/N)` is a safe substituion for `ORDER BY random() LIMIT n` only when you don't need a sample that's guaranteed to be from a uniform distribution of rows. That's because TABLESAMPLE SYSTEM samples blocks, not individual rows: > The SYSTEM method does block-level sampling with each block having the specified chance of being selected; all rows in each selected block are returned.