Apparently PostgreSQL stores a couple of values in the header of each database row.
If I don't use NULL values in that table - is the null bitmap still there?
Does defining the columns with NOT NULL make any difference?
The null bitmap needs one bit per column in the row, rounded up to full bytes. It is only there if the actual row includes at least one null value and is fully allocated in that case. NOT NULL constraints do not directly affect that. (Of course, if all fields of your table are defined NOT NULL, there can never be a null bitmap.)
The "heap tuple header" (per row) is 23 bytes long. Actual data starts at a multiple of MAXALIGN (Maximum data alignment) after that, which is typically 8 bytes on 64-bit OS (4 bytes on 32-bit OS). Run the following command from your PostgreSQL binary dir as root to get a definitive answer:
./pg_controldata /path/to/my/dbcluster
On a typical Debian-based installation of Postgres 17 that would be:
sudo /usr/lib/postgresql/17/bin/pg_controldata /var/lib/postgresql/17/main
Either way, there is one free byte between the header and the aligned start of the data, which the null bitmap can utilize. As long as your table has 8 columns or less, null storage is effectively absolutely free (as far as disk space is concerned).
After that, another MAXALIGN (typically 8 bytes) is allocated for the null bitmap to cover another (typically) 64 fields. Etc.
This is valid for at least versions 8.4 - 17 and most likely won't change.