Postgres uses 1 bit per NULL value, though the reality is more complex as everything is padded for alignment. So fewer than 8 nullable columns are free, and abo

Postgres uses 1 bit per NULL value, though the reality is more complex as everything is padded for alignment. So fewer than 8 nullable columns are free, and above that you pay probably 8 bytes at once (I'm not totally sure on the exact numbers here). So for Postgres it is generally true that storing NULLs is very cheap in most cases.