This problem have been noticed many times.

PostgreSQL 7.* doesn't allow "SET NOT NULL" to column that already eixists.

Many MODs use the file /include/db/db_tools.php which contains the code below

    if ($column_data['null'] == 'NOT NULL')

    {

    $statements[] = 'ALTER TABLE ' . $table_name . ' ALTER COLUMN ' . $column_name . ' SET NOT NULL';

    }

We can install some MODs only together with the core system. Or by commenting this strings.