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[https://www.postgresql.org/ PostgreSQL] also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance.


== Using <code>psql</code> ==
This article extends the documentation in the [https://nixos.org/manual/nixos/stable/#module-postgresql NixOS manual].


Depending on the <code>system.stateVersion</code> of your system, the [https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/databases/postgresql.nix#L151 default super-user username] will change:
=== Getting started ===


* 17.03 and earlier: the superuser is <code>root</code>
To try out Postgresql add the following minimal example to your [[NixOS modules | NixOS module]]:
* 17.09 and later: the superuser is <code>postgres</code>


There is no password, [https://www.postgresql.org/docs/9.6/static/auth-methods.html#AUTH-IDENT Ident Authentication] is used. This means that you can access the database using a system user named like the database user.
<syntaxhighlight lang="nix">
{
  # ...
  config.services.postgresql = {
    enable = true;
    ensureDatabases = [ "mydatabase" ];
    authentication = pkgs.lib.mkOverride 10 ''
      #type database  DBuser  auth-method
      local all      all    trust
    '';
  };
} 
</syntaxhighlight>
 
This will setup Postgresql with a default DB superuser/admin "postgres", a database "mydatabase" and let every DB user have access to it without a password through a "local" Unix socket "/var/lib/postgresql" (TCP/IP is disabled by default because it's less performant and less secure).
 
* [https://search.nixos.org/options?query=services.postgresql Available NixOS Postgresql service options]
It's also possible to setup PostgreSQL with [[Nix Darwin]] similar to how you would on NixOS, see the [https://daiderd.com/nix-darwin/manual/index.html#opt-services.postgresql.enable options].
 
=== Verify  setup ===
You can use <code>psql</code> that comes with Postgres in the terminal to verify that the DB setup is as expected:
 
<syntaxhighlight lang="bash">
$ sudo -u postgres psql
psql
Type "help" for help.
 
postgres=#
</syntaxhighlight>


Example for a 17.03 stateVersion.
We have to switch to a system user like "postgres" with <code>sudo -u postgres</code>, because by default <code>psql</code> logs you into the DB user of the same name as the current Linux/system user. By default, NixOS creates a system and DB user names "postgres".
So the line <code>postgres=# </code> shows that we are now logged-in as DB user "postgres".


<syntaxhighlight lang="commands">
Inside <code>psql</code> here the most frequent commands are:
$ sudo -u root psql
 
psql: FATAL:  database "root" does not exist
List all databases running on this Postgres instance with <code>\l</code>:
$ sudo -u root psql -l
 
                              List of databases
<syntaxhighlight lang="bash">
  Name    | Owner | Encoding |  Collate  |    Ctype    | Access privileges
postgres=# \l
-----------+-------+----------+-------------+-------------+-------------------
                                  List of databases
  postgres  | root  | UTF8    | en_CA.UTF-8 | en_CA.UTF-8 |
    Name    | Owner   | Encoding |  Collate  |    Ctype    |   Access privileges  
  template0 | root  | UTF8    | en_CA.UTF-8 | en_CA.UTF-8 | =c/root          +
------------+----------+----------+-------------+-------------+------------------------
          |       |          |            |            | root=CTc/root
  mydatabase | postgres | UTF8    | en_US.UTF-8 | en_US.UTF-8 | =Tc/postgres          +
  template1 | root  | UTF8    | en_CA.UTF-8 | en_CA.UTF-8 | =c/root          +
            |          |          |            |            | postgres=CTc/postgres +
          |       |          |            |            | root=CTc/root
            |          |          |            |            | rustnixos=CTc/postgres
  postgres  | postgres | UTF8    | en_US.UTF-8 | en_US.UTF-8 |  
  template0 | postgres | UTF8    | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +
            |         |          |            |            | postgres=CTc/postgres
  template1 | postgres | UTF8    | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +
            |         |          |            |            | postgres=CTc/postgres
(4 rows)
(4 rows)
</syntaxhighlight>
</syntaxhighlight>


The first error is not an error with the credentials, but an error coming from the default behaviour of <code>psql</code> that is trying to use a database name named like the user logging-in. The second command lists tables available.
List all DB users (also called "roles" in Postgres) with <code>\du</code>:
 
<syntaxhighlight lang="bash">
postgres=# \du
                                  List of roles
Role name |                        Attributes                        | Member of
-----------+------------------------------------------------------------+-----------
postgres  | Superuser, Create role, Create DB, Replication, Bypass RLS | {}
</syntaxhighlight>
 
List all authentication rules (called an "pg_hba.conf" file in Postgres ) with <code>table pg_hba_file_rules;</code>:
 
<syntaxhighlight lang="bash">
postgres=# table pg_hba_file_rules;
line_number | type  | database | user_name | address | netmask | auth_method | options | error  
-------------+-------+----------+-----------+---------+---------+-------------+---------+-------
          1 | local | {all}    | {all}    |        |        | peer        |        |
(1 row)
</syntaxhighlight>
 
* [https://www.postgresql.org/docs/current/auth-pg-hba-conf.html Official Postgres authentication pg_hba.conf documentation]
 
=== Allow TCP/IP connections ===
 
This example shows how to roll out a database with a default user and password:
 
<syntaxhighlight lang="nix">
services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
  enableTCPIP = true;
  # port = 5432;
  authentication = pkgs.lib.mkOverride 10 ''
    #...
    #type database DBuser origin-address auth-method
    # ipv4
    host  all      all    127.0.0.1/32  trust
    # ipv6
    host all      all    ::1/128        trust
  '';
  initialScript = pkgs.writeText "backend-initScript" ''
    CREATE ROLE nixcloud WITH LOGIN PASSWORD 'nixcloud' CREATEDB;
    CREATE DATABASE nixcloud;
    GRANT ALL PRIVILEGES ON DATABASE nixcloud TO nixcloud;
  '';
};
</syntaxhighlight>
 
This will allow "host" based authentification only from other webservices on the same computer ("127.0.0.1"), although any DB user will have access to any database.
 
=== Set the Postgresql versions ===
 
By default, NixOS uses whatever Postgres version shipped as default for your [https://search.nixos.org/options?show=system.stateVersion system.stateVersion]. To use a different or more recent version, you can set it manually:
 
<syntaxhighlight lang="nix">
services.postgresql = {
  enable = true;
  package = pkgs.postgresql_15;
  # ...
};
</syntaxhighlight>
 
Note that changing the package version does not trigger any automatic migrations of your existing databases: if you update Postgres you should be ready to migrate any existing databases manually.
 
* [https://search.nixos.org/packages?query=postgresql_ Available Nixpkgs Postgresql versions]
 
=== Security ===
 
Letting every system and DB user have access to all data is dangerous. Postgres supports several layers of protection.
One is to '''prefer "local" connections using Unix sockets''', that aren't accessible from the internet, whenever Postgres and your client app run on the same server.
 
==== Harden authentication  ====
 
We can '''limit what system user can connect'''.
 
Postgres supports [https://www.postgresql.org/docs/current/auth-username-maps.html "user name maps"], which limit which system users can log in as which DB user:
 
<syntaxhighlight lang="nix">
services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
  identMap = ''
    # ArbitraryMapName systemUser DBUser
      superuser_map      root      postgres
      superuser_map      postgres  postgres
      # Let other names login as themselves
      superuser_map      /^(.*)$  \1
  '';
};
</syntaxhighlight>
 
This map can have an arbitrary name and defines which system user can login as which DB user. Every other user and combination is rejected.
For example, with the above mapping if we are logged-in as system user "root" but want enter the DB as DB user "postgres" we would be allowed:
 
<syntaxhighlight lang="nix">
root$ psql -U postgres
# ok
</syntaxhighlight>
 
==== Limit Access  ====
 
Once logged-in we can '''limit what DB users can access'''. With the <code>authentication</code> we can limit what
DB user can access which databases. A good default setting is as follows:
 
<syntaxhighlight lang="nix">
services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
  authentication = pkgs.lib.mkOverride 10 ''
    #type database  DBuser  auth-method optional_ident_map
    local sameuser  all    peer        map=superuser_map
  '';
};
</syntaxhighlight>
 
With "sameuser"  Postgres will allow DB user access only to databases of the same name. E.g. DB user "mydatabase" will get access to database "mydatabase" and nothing else. The part <code>map=superuser_map</code> is optional.
One exception is the DB user "postgres", which by default is a superuser/admin with access to everything.
 
== TLS ==
 
To turn TLS on in recent versions of postgres it's pretty easy. Their [https://www.postgresql.org/docs/current/ssl-tcp.html docs] are pretty good.
 
Create a simple cert just to make it work. If you are doing this in production, you need to provide your own server.crt and server.key in the main PGDATA dir (~postgres).
 
In a shell:
<syntaxhighlight lang="nix">
cd ~postgres
sudo -u postgres openssl req -new -x509 -days 365 -nodes -text -out server.crt  -keyout server.key -subj "/CN=dbhost.yourdomain.com"
chmod og-rwx server.key
</syntaxhighlight>
 
Then in your nix configuration:
 
<syntaxhighlight lang="nix">
  services.postgresql = {
    enable = true;
    package = pkgs.postgresql_16;
    enableTCPIP = true;
    ensureDatabases = [ "tootieapp" ];
    settings = {
        ssl = true;
    };
    authentication = pkgs.lib.mkOverride 10 ''
      #type database  DBuser  auth-method
      local all      all    trust
      host  sameuser    all    127.0.0.1/32 scram-sha-256
      host  sameuser    all    ::1/128 scram-sha-256
    '';
  };
</syntaxhighlight>
 
the `sameuser` mentioned in the authentication section requires the database name be the same as the username, which you may not want, you can change that to `all` to allow an authenticated user the ability to connect to any database.
 
`scram-sha-256` is the require a password option, but you can authenticate a variety of different ways, see the official docs for other options as part of pg_hba.conf.
 
user creation and permissions are best described in the PG manual under `CREATE ROLE` and `GRANT`
for example:
<syntaxhighlight lang="sql">
CREATE USER tootieapp WITH PASSWORD 'BIGLONGRANDOMSTRINGHERE';
GRANT ALL PRIVILEGES ON DATABASE tootieapp TO tootieapp;
</syntaxhighlight>
 
 
== Debugging with <code>psql</code> ==
 
To debug the SQL statements futher, one can use '''systemctl cat postgresql''' and see the '''ExecStartPost=/nix/store/rnv1v95bbf2lsy9ncwg7jdwj2s71sqra-unit-script/bin/postgresql-post-start''' line. Then open it with `cat` on the shell and see the '''psql''' command.
 
 
Then execute the complete statement on the shell, as:
 
<pre>
/nix/store/3mqha1naji34i6iv78i90hc20dx0hld9-sudo-1.8.20p2/bin/sudo -u postgres psql -f "/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript" --port=5432 -d postgres
psql:/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript:1: ERROR:  syntax error at or near "-"
LINE 1: CREATE ROLE nixcloud-admin WITH LOGIN PASSWORD 'nixcloud' CR...
                            ^
psql:/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript:2: ERROR:  database "nixcloud-db1" already exists
psql:/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript:3: ERROR:  syntax error at or near "-"
LINE 1: ...ALL PRIVILEGES ON DATABASE "nixcloud-db1" TO nixcloud-admin;
                                                                ^
</pre>
 
== Troubleshooting ==
 
=== Connection rejected with "Role does not exist" ===
 
<syntaxhighlight lang="nix">
$ psql
psql: error: connection to server on socket "/run/postgresql/.s.PGSQL.5432" failed: FATAL:  role "root" does not exist
</syntaxhighlight>
 
You are trying to login as a system user ("root" in this example) that has no DB user of the same name. Try <code>psql -U postgres</code> or <code>sudo -u postgres psql</code> to log in as a different DB user.
 
=== Connection rejected with "Peer authentication failed" ===
 
<syntaxhighlight lang="nix">
root$ psql -U postgres
psql: error: connection to server on socket "/run/postgresql/.s.PGSQL.5432" failed: FATAL:  Peer authentication failed for user "postgress"
</syntaxhighlight>
 
You are trying to login as a DB user ("postgres" in this example) for which your current system user ("root" in this example) has no permission to switch to. Check your "user name map" in the <code>identMap</code> section.
 
=== WARNING: database "XXX" has a collation version mismatch ===
 
The complete error which appears in the system log might look similar to this
 
<syntaxhighlight>
WARNING:  database "outline" has a collation version mismatch
DETAIL:  The database was created using collation version 2.35, but the operating system provides version 2.38.
HINT:  Rebuild all objects in this database that use the default collation and run ALTER DATABASE outline REFRESH COLLATION VERSION, or build PostgreSQL with the right library version.
</syntaxhighlight>
 
To fix it, run following commands in the psql console. Replace the database name <code>outline</code> with the name of the database which you want to migrate
 
<syntaxhighlight>
sudo -u postgres psql
postgres=# \c outline;
outline=# REINDEX DATABASE outline;
outline=# ALTER DATABASE outline REFRESH COLLATION VERSION;
</syntaxhighlight>
== See also ==
 
* [https://search.nixos.org/options?query=services.postgresql Available NixOS service options]
 
 
[[Category:Applications]]
[[Category:Database]]
[[Category:NixOS Manual]]
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