Postgresql restricts the operation of a user to only connect to a certain database

Create a database bbb and owner is user b:

postgres9.6@[local]:5432 postgres# create database bbb owner b;
CREATE DATABASE
Time: 259.099 ms

By default, you can also connect to the database bbb using user c:

postgres9.6@[local]:5432 postgres# \c bbb c
You are now connected to database "bbb" as user "c".

Recycling public's connect on database bbb permissions:

c@[local]:5432 bbb# \c postgres postgres9.6
You are now connected to database "postgres" as user "postgres9.6".
postgres9.6@[local]:5432 postgres# revoke connect on database bbb from public;
REVOKE
Time: 2.088 ms

At this time, user c does not have permission to connect to the database bbb:

postgres9.6@[local]:5432 postgres# \c bbb c
FATAL: permission denied for database "bbb"
DETAIL: User does not have CONNECT privilege.
Previous connection kept

But the owner user b of the bbb database can connect to the database:

postgres9.6@[local]:5432 postgres# \c bbb b
You are now connected to database "bbb" as user "b".
b@[local]:5432 bbb#

In this case, the superuser can also connect to the database:

b@[local]:5432 bbb# \c bbb postgres9.6
You are now connected to database "bbb" as user "postgres9.6".
postgres9.6@[local]:5432 bbb# 
postgres9.6@[local]:5432 bbb# \du
                  List of roles
 Role name |             Attributes             | Member of 
-------------+------------------------------------------------------------+-----------
 a      |                              | {}
 b      |                              | {}
 c      |                              | {}
 postgres9.6 | Superuser, Create role, Create DB, Replication, Bypass RLS | {}

Another method: define from pg_hba.conf:

 # TYPE DATABASE    USER      ADDRESS         METHOD
# "local" is for Unix domain socket connections only
local  all       all                  trust 
# IPv4 local connections:
host  all       all       127.0.0.1/32      trust
...
#
host  all  all        0.0.0.0/0       md5

Additional: Postgres limits each user to only connect to a specified number of sessions to prevent server resources from being tight

Limit each user to connect to a specified number of sessions to prevent server resources from being tight

(1) Create a test user:

highgo=#create user test;
CREATEROLE
highgo=#\du
               List of roles
 Role name |          Attributes          | Member of
-----------+------------------------------------------------+----------
 highgo  | Superuser, Create role, Create DB, Replication | {}
 test   |                        | {}

(2) Setting only allows users to use one connection by test

highgo=#ALTER ROLE test CONNECTION LIMIT 1;
ALTERROLE

(3) Use the test user to connect to the highgo database in session 1

highgo=>\c highgo test
Youare now connected to database "highgo" as user "test".
highgo=>

(4) In session 2, use the test user to connect to the highgo database, and the following error will appear:

highgo=#\c highgo test
Fatal error: By the role"test"Too many connections initiated
Previousconnection kept

(5) Query user test link restrictions

highgo=>SELECT rolconnlimit FROM pg_roles WHERE rolname = 'test';
 rolconnlimit
--------------
      1
(1row)

The above is personal experience. I hope you can give you a reference and I hope you can support me more. If there are any mistakes or no complete considerations, I would like to give you advice.