How to connect to PostgreSQL? - postgresql

Question

I want to connect to a PostgreSQL server with rust-postgres:
let mut client = Client::connect("host=localhost user=postgres", NoTls)?;
The complete code example is from Client and Config.
I keep getting the error
Error: Error { kind: Connect, cause: Some(Os { code: 111, kind: ConnectionRefused, message: "Connection refused" }) }
In a terminal, I can interact with PostgreSQL:
(base) wm#wm:~/Desktop/HP$ sudo -i -u postgres
postgres#wm:~$ psql
psql (10.10 (Ubuntu 10.10-0ubuntu0.18.04.1))
Type "help" for help.
postgres=#
There are few issues relevant to failed connections, so I guess there must be something I missed.

Answer 1

Postgresql supports client connections via local Unix sockets or via TCP/IP connections.
In the default configuration, though, it will not listen for TCP/IP connections. It will only listen for connections to a local Unix socket. The location of this socket is defined by the postgresql configuration variable unix_socket_directories.
In your tests, you have found that when running the psql command line tool with no arguments, it manages to connect to the database. This works because psql uses the postgresql supplied client library (libpq) and this client library has the default behavior of connecting to the local Unix socket if no hostname is supplied.
However, when using rust-postgres, you are supplying a connection string including the text "host=localhost". This is instructing rust-postgres to make a connection to IP address localhost. It fails because the postgresql server is not configured to listen on any IP interfaces, and only listen on a Unix socket.
You could change the connection string to specify the unix socket to connect to, for example:
host=/var/run/postgresql/.s.PGSQL.5432
The value above reflects where the socket is on my machine, it might be different on yours.
The libpq library allows you to leave the host parameter out altogether and it will connect to the Unix socket automatically; unfortunately it does not look like rust-postgres supports this though.
Alternatively, you could reconfigure your postgresql server so that it listens on the loopback IP adapter. This involves setting the listen_addresses parameter in the postgresl configuration file. See this answer for more details, including changes you will need to make to the authentication in pg_hba.conf.