cannot connect to PostgreSQL DB running on EC2 instance

I have a simple PostgreSQL DB running on an EC2 instance.

ubuntu@ip-172-31-38-xx:~$ service postgresql status
● postgresql.service - PostgreSQL RDBMS
   Loaded: loaded (/lib/systemd/system/postgresql.service; enabled; vendor preset: enabled)
   Active: active (exited) since Fri 2020-06-19 14:04:12 UTC; 7h ago
 Main PID: 11065 (code=exited, status=0/SUCCESS)
    Tasks: 0 (limit: 1152)
   CGroup: /system.slice/postgresql.service

Jun 19 14:04:12 ip-172-31-38-xx systemd[1]: Starting PostgreSQL RDBMS...
Jun 19 14:04:12 ip-172-31-38-xx systemd[1]: Started PostgreSQL RDBMS.

ubuntu@ip-172-31-38-xx:~$ psql -U postgres
Password for user postgres:
psql (10.12 (Ubuntu 10.12-0ubuntu0.18.04.1))
Type "help" for help.

postgres=# SELECT *
postgres-# FROM pg_settings
postgres-# WHERE name = 'port';
 name | setting | unit |                       category                       |                short_desc                | extra_desc |  context   | vartype |       source       | min_val | max_val | enumvals | boot_val | reset_val |               sourcefile                | sourceline | pending_restart
------+---------+------+------------------------------------------------------+------------------------------------------+------------+------------+---------+--------------------+---------+---------+----------+----------+-----------+-----------------------------------------+------------+-----------------
 port | 5432    |      | Connections and Authentication / Connection Settings | Sets the TCP port the server listens on. |            | postmaster | integer | configuration file | 1       | 65535   |          | 5432     | 5432      | /etc/postgresql/10/main/postgresql.conf |         63 | f
(1 row)

The only Security Group that is associated with this EC2 instance has inbound rules wide open:

5432, TCP, 0.0.0.0/0

But when I use a client to connect to this DB with the correct hostname (public IP/DNS), port number, DB name, user name and password typed in, it always says:

could not connect to server: Connection refused, is the server running on host "ec2-dns.com(172.public.ip)" and accepting TCP/IP connections on port 5432?

All right, I've figured it out from this answer

Two things I did to enable myself to connect (exactly from the link above, I'm duplicating it here for convenience):

1. open this file: sudo vi /etc/postgresql/10/main/pg_hba.conf

immediately below this line:

host all all 127.0.0.1/32 md5

added this line:

host all all 0.0.0.0/0 md5

2. open this file: sudo vi /etc/postgresql/10/main/postgresql.conf

find a line that starts with this: #listen_addresses = 'localhost'

Uncomment the line by deleting the #, and change 'localhost' to '*'.

The line should now look like this:

listen_addresses = '*' # what IP address(es) to listen on;.

then restart your service:

sudo service postgresql restart

then you should be able to connect to your DB via a SQL client.