Supabase has reached General Availability

Learn more

Postgres Roles and Privileges

2024-04-11

43 minute read

Controlling access to data in Postgres is paramount for data security. Postgres provides a robust and flexible permissions model for users to manage access to their data. The permissions model is based on the familiar object, privilege, role model but has subtleties which must be understood by a database administrator to create airtight access. In this post we will take a detailed look at how roles and permissions work in Postgres.

Basic Concepts

Let's first understand some basic concepts which will be used throughout the rest of the post.

Database Object

A database object is any entity created in the database. Tables, foreign tables, views, materialized views, types, domains, operators, functions, triggers etc. are database objects. Objects allow operations on them which vary for each object. For example, you can select data from a table and you can execute a function.

Privilege

A privilege controls what operation is allowed to be run on a database object. For example, the select privilege on a table controls the ability to read data from the table. Similarly, the execute privilege controls the ability to execute a function. Privileges are assigned to roles. A role must have the permission for the operation it is performing on an object.

Role

A role is a user or a group. A user is someone who can login to the database. A group is a collection of users to make it easier to manage privileges for users. Unlike a user, a group can't login to the database. The distinction between a user and a group doesn’t matter to Postgres for the most part as they are both roles, but it is still useful to think of them as separate concepts for ease of understanding.

Owner

Every database object has an owner. The owner has complete control over the object. They can modify or delete the object or grant privileges to other users and groups. When a user creates a new object, they become the owner of the object. An owner can also transfer the ownership of objects to other roles. A role cannot be deleted before all its owned objects’ ownership is transferred to another role.

With these basic terms defined, let's take a look at the permissions model in Postgres in depth. The rest of the post will be more like a tutorial, so you can follow along. I'll be using a hosted Supabase project, but you are free to use any Postgres installation.

Setting Up

Create a new Supabase project (or use an existing one) and copy its connection string URI from the Database Settings page. The URI looks like the following:


_10
postgres://[USER].[YOUR-PROJECT-REF]:[YOUR-PASSWORD]@[REGION-SUBDOMAIN].pooler.supabase.com:5432/postgres

Where USER is the user to connect as. YOUR-PROJECT-REF is a string uniquely identifying your project. YOUR-PASSWORD is the database password for the USER user and REGION-SUBDOMAIN is the subdomain where your database is hosted.

Use the psql command line tool to connect to the database:


_10
➜ psql postgres://postgres.[YOUR-PROJECT-REF]:[YOUR-PASSWORD]@[REGION-SUBDOMAIN].pooler.supabase.com:5432/postgres

Once connected, confirm that you are connected as the postgres user by running select current_role command:


_10
# as postgres
_10
postgres=> select current_role;
_10
┌──────────────┐
_10
│ current_role │
_10
├──────────────┤
_10
│ portgres │
_10
└──────────────┘
_10
(1 row)

Creating Roles

Now, let's create two users named junior_dev and senior_dev. A database role can be created with the create role command. Since a user is a role that can login, use the login parameter:


_10
# as postgres
_10
postgres=> create role junior_dev login password 'a long and secure password';
_10
CREATE ROLE
_10
postgres=> create role senior_dev login password 'another long and secure password';
_10
CREATE ROLE

You can now confirm that the junior_dev and senior_dev users can login to the database:


_21
➜ psql postgres://junior_dev.[YOUR-PROJECT-REF]:[YOUR-PASSWORD]@[REGION-SUBDOMAIN].pooler.supabase.com:5432/postgres
_21
_21
postgres=> select current_role;
_21
┌──────────────┐
_21
│ current_role │
_21
├──────────────┤
_21
│ junior_dev │
_21
└──────────────┘
_21
(1 row)
_21
_21
postgres=> exit
_21
_21
➜ psql postgres://senior_dev.[YOUR-PROJECT-REF]:[YOUR-PASSWORD]@[REGION-SUBDOMAIN].pooler.supabase.com:5432/postgres
_21
_21
postgres=> select current_role;
_21
┌──────────────┐
_21
│ current_role │
_21
├──────────────┤
_21
│ senior_dev │
_21
└──────────────┘
_21
(1 row)

For the rest of the post, open three terminals and login each with junior_dev, senior_dev and postgres to easily switch between them. Each executed command will list at the beginning the user it should be executed as, for example:


_10
# as junior_dev
_10
postgres=> this command should be executed from the junior_dev's terminal

Creating Objects and Assigning Privileges

Let's now try to create a table from as junior_dev:


_10
# as junior_dev
_10
postgres=> create table public.apps(id serial primary key, name text);
_10
ERROR: permission denied for schema public
_10
LINE 1: create table public.apps(id serial primary key, name text);
_10
^

What happened? The error permission denied for schema public tells us that junior_dev doesn't have some permission on the public schema. We can check existing permissions on a schema using the dn+ <schema> command in psql:


_14
# as junior_dev
_14
postgres=> \dn+ public
_14
List of schemas
_14
┌────────┬───────────────────┬────────────────────────────────────────┬────────────────────────┐
_14
│ Name │ Owner │ Access privileges │ Description │
_14
├────────┼───────────────────┼────────────────────────────────────────┼────────────────────────┤
_14
│ public │ pg_database_owner │ pg_database_owner=UC/pg_database_owner↵│ standard public schema │
_14
│ │ │ =U/pg_database_owner ↵│ │
_14
│ │ │ postgres=U/pg_database_owner ↵│ │
_14
│ │ │ anon=U/pg_database_owner ↵│ │
_14
│ │ │ authenticated=U/pg_database_owner ↵│ │
_14
│ │ │ service_role=U/pg_database_owner │ │
_14
└────────┴───────────────────┴────────────────────────────────────────┴────────────────────────┘
_14
(1 row)

Indeed, the Access privileges column doesn’t list junior_dev role anywhere, which means it doesn’t have any permission on the public schema. How do we fix this? Thepostgres user in Supabase hosted databases is a powerful role with more privileges than many other roles. Think of the postgres role as an admin role, although it is not a superuser. We can use this role to grant appropriate permissions.

So, let’s switch to the postgres user connection and grant junior_dev the permission to create objects in the public schema. The general format of the grant command is grant <privilege> on <object> to <role>. You can consult the privileges page in Postgres documentation to find out the correct privilege name.


_10
# as postgres
_10
postgres=> grant create on schema public to junior_dev;
_10
GRANT

Let’s check the permissions again:


_15
# as junior_dev
_15
postgres=> \dn+ public
_15
List of schemas
_15
┌────────┬───────────────────┬────────────────────────────────────────┬────────────────────────┐
_15
│ Name │ Owner │ Access privileges │ Description │
_15
├────────┼───────────────────┼────────────────────────────────────────┼────────────────────────┤
_15
│ public │ pg_database_owner │ pg_database_owner=UC/pg_database_owner↵│ standard public schema │
_15
│ │ │ =U/pg_database_owner ↵│ │
_15
│ │ │ postgres=U/pg_database_owner ↵│ │
_15
│ │ │ anon=U/pg_database_owner ↵│ │
_15
│ │ │ authenticated=U/pg_database_owner ↵│ │
_15
│ │ │ service_role=U/pg_database_owner ↵│ │
_15
│ │ │ junior_dev=C/pg_database_owner │ │
_15
└────────┴───────────────────┴────────────────────────────────────────┴────────────────────────┘
_15
(1 row)

This time we see a new line in the access privileges column:


_10
# as junior_dev
_10
postgres=> create table public.apps(id serial primary key, name text);
_10
CREATE TABLE

Let’s insert some data in it:


_10
# as junior_dev
_10
postgres=> insert into public.apps(name) values ('next app');
_10
INSERT 0 1
_10
postgres=> select * from public.apps;
_10
┌────┬──────────┐
_10
│ id │ name │
_10
├────┼──────────┤
_10
│ 1 │ next app │
_10
└────┴──────────┘
_10
(1 row)

Now switch to senior_dev and try to select data from the table:


_10
# as senior_dev
_10
postgres=> select * from public.apps;
_10
ERROR: permission denied for table apps

senior_dev can’t select data from the public.apps table. Let’s debug the permissions error as before. The command in psql to view table permissions is \dp <tablename>:


_10
# as senior_dev
_10
postgres=> \dp public.apps
_10
Access privileges
_10
┌────────┬──────┬───────┬───────────────────┬───────────────────┬──────────┐
_10
│ Schema │ Name │ Type │ Access privileges │ Column privileges │ Policies │
_10
├────────┼──────┼───────┼───────────────────┼───────────────────┼──────────┤
_10
│ public │ apps │ table │ │ │ │
_10
└────────┴──────┴───────┴───────────────────┴───────────────────┴──────────┘
_10
(1 row)

No access privileges are present at all. As we did before, let’s now switch to the postgres user and fix the permissions. The privileges page tells us that we need to grant the select privilege to senior_dev for them to select data from the public.apps table:


_10
# as postgres
_10
postgres=> grant select on table public.apps to senior_dev;
_10
ERROR: permission denied for table apps

Why can’t postgres grant the select privilege? Because it is neither an owner, nor has it any access privileges on the table. But then how was junior_dev able to select data from the table? That is because junior_dev is the owner of the table:


_10
# as postgres
_10
postgres=> \dt public.apps
_10
List of relations
_10
┌────────┬──────┬───────┬────────────┐
_10
│ Schema │ Name │ Type │ Owner │
_10
├────────┼──────┼───────┼────────────┤
_10
│ public │ apps │ table │ junior_dev │
_10
└────────┴──────┴───────┴────────────┘
_10
(1 row)

Since an owner has all the privileges on an object, junior_dev can select the data. junior_dev can also grant privileges on the owned objects to other roles. Let’s fix the permissions with junior_dev:


_10
# as junior_dev
_10
postgres=> grant select on public.apps to senior_dev;
_10
GRANT

Now senior_dev can select the data:


_10
# as senior_dev
_10
postgres=> select * from public.apps;
_10
┌────┬──────────┐
_10
│ id │ name │
_10
├────┼──────────┤
_10
│ 1 │ next app │
_10
└────┴──────────┘
_10
(1 row)

Another option in the above example would have been for junior_dev to grant the privilege to grant the select privilege to the postgres role. The postgres role would then have been able to grant the select privilege to senior_dev. To try this, let’s revoke the previously granted privilege to senior_dev first:


_10
# as junior_dev
_10
postgres=> revoke select on public.apps from senior_dev;
_10
REVOKE

And then grant the select privilege with grant option to postgres:


_10
# as junior_dev
_10
postgres=> grant select on public.apps to postgres with grant option;
_10
GRANT

Now, if we view the permissions on the public.apps table:


_10
# as_junior_dev
_10
postgres=> \dp public.apps
_10
Access privileges
_10
┌────────┬──────┬───────┬───────────────────────────────┬───────────────────┬──────────┐
_10
│ Schema │ Name │ Type │ Access privileges │ Column privileges │ Policies │
_10
├────────┼──────┼───────┼───────────────────────────────┼───────────────────┼──────────┤
_10
│ public │ apps │ table │ junior_dev=arwdDxt/junior_dev↵│ │ │
_10
│ │ │ │ postgres=r*/junior_dev │ │ │
_10
└────────┴──────┴───────┴───────────────────────────────┴───────────────────┴──────────┘
_10
(1 row)

Notice the * after the r in postgres=r*/junior_dev. which indicates that the select permission was granted with grant option. Now postgres can grant the select privilege to senior_dev:


_10
# as postgres
_10
postgres=> grant select on table public.apps to senior_dev;
_10
GRANT

And senior_dev has the select privilege and can select from the table again:


_19
# as senior_dev
_19
postgres=> \dp public.apps
_19
Access privileges
_19
┌────────┬──────┬───────┬───────────────────────────────┬───────────────────┬──────────┐
_19
│ Schema │ Name │ Type │ Access privileges │ Column privileges │ Policies │
_19
├────────┼──────┼───────┼───────────────────────────────┼───────────────────┼──────────┤
_19
│ public │ apps │ table │ junior_dev=arwdDxt/junior_dev↵│ │ │
_19
│ │ │ │ postgres=r*/junior_dev ↵│ │ │
_19
│ │ │ │ senior_dev=r/postgres │ │ │
_19
└────────┴──────┴───────┴───────────────────────────────┴───────────────────┴──────────┘
_19
(1 row)
_19
_19
postgres=> select * from public.apps;
_19
┌────┬──────────┐
_19
│ id │ name │
_19
├────┼──────────┤
_19
│ 1 │ next app │
_19
└────┴──────────┘
_19
(1 row)

A grant command only adds privileges for existing objects. What if we want to grant certain privileges to objects as soon as they are created? That’s where default access privileges come in.

Default Access Privileges

If junior_dev now creates another table, it has to grant the privileges again to senior_dev. To avoid doing this each time junior_dev creates a new table, we can alter junior_dev's default access privileges. First let’s see the current default privileges on the public schema:


_32
# as junior_dev
_32
postgres=> \ddp public
_32
Default access privileges
_32
┌────────────────┬────────┬──────────┬──────────────────────────────────────┐
_32
│ Owner │ Schema │ Type │ Access privileges │
_32
├────────────────┼────────┼──────────┼──────────────────────────────────────┤
_32
│ postgres │ public │ function │ postgres=X/postgres ↵│
_32
│ │ │ │ anon=X/postgres ↵│
_32
│ │ │ │ authenticated=X/postgres ↵│
_32
│ │ │ │ service_role=X/postgres │
_32
│ postgres │ public │ sequence │ postgres=rwU/postgres ↵│
_32
│ │ │ │ anon=rwU/postgres ↵│
_32
│ │ │ │ authenticated=rwU/postgres ↵│
_32
│ │ │ │ service_role=rwU/postgres │
_32
│ postgres │ public │ table │ postgres=arwdDxt/postgres ↵│
_32
│ │ │ │ anon=arwdDxt/postgres ↵│
_32
│ │ │ │ authenticated=arwdDxt/postgres ↵│
_32
│ │ │ │ service_role=arwdDxt/postgres │
_32
│ supabase_admin │ public │ function │ postgres=X/supabase_admin ↵│
_32
│ │ │ │ anon=X/supabase_admin ↵│
_32
│ │ │ │ authenticated=X/supabase_admin ↵│
_32
│ │ │ │ service_role=X/supabase_admin │
_32
│ supabase_admin │ public │ sequence │ postgres=rwU/supabase_admin ↵│
_32
│ │ │ │ anon=rwU/supabase_admin ↵│
_32
│ │ │ │ authenticated=rwU/supabase_admin ↵│
_32
│ │ │ │ service_role=rwU/supabase_admin │
_32
│ supabase_admin │ public │ table │ postgres=arwdDxt/supabase_admin ↵│
_32
│ │ │ │ anon=arwdDxt/supabase_admin ↵│
_32
│ │ │ │ authenticated=arwdDxt/supabase_admin↵│
_32
│ │ │ │ service_role=arwdDxt/supabase_admin │
_32
└────────────────┴────────┴──────────┴──────────────────────────────────────┘
_32
(6 rows)

Neither junior_dev nor senior_dev are listed. Let’s alter junior_dev's default privileges:


_10
# as junior_dev
_10
postgres=> alter default privileges in schema public grant select on tables to senior_dev;
_10
ALTER DEFAULT PRIVILEGES

Here we are altering default privileges such that whenever junior_dev creates a new table in the public schema, senior_dev should be granted select privilege on it. Let’s check the privileges again:


_33
# as junior_dev
_33
postgres=> \ddp public
_33
Default access privileges
_33
┌────────────────┬────────┬──────────┬──────────────────────────────────────┐
_33
│ Owner │ Schema │ Type │ Access privileges │
_33
├────────────────┼────────┼──────────┼──────────────────────────────────────┤
_33
│ junior_dev │ public │ table │ senior_dev=r/junior_dev │
_33
│ postgres │ public │ function │ postgres=X/postgres ↵│
_33
│ │ │ │ anon=X/postgres ↵│
_33
│ │ │ │ authenticated=X/postgres ↵│
_33
│ │ │ │ service_role=X/postgres │
_33
│ postgres │ public │ sequence │ postgres=rwU/postgres ↵│
_33
│ │ │ │ anon=rwU/postgres ↵│
_33
│ │ │ │ authenticated=rwU/postgres ↵│
_33
│ │ │ │ service_role=rwU/postgres │
_33
│ postgres │ public │ table │ postgres=arwdDxt/postgres ↵│
_33
│ │ │ │ anon=arwdDxt/postgres ↵│
_33
│ │ │ │ authenticated=arwdDxt/postgres ↵│
_33
│ │ │ │ service_role=arwdDxt/postgres │
_33
│ supabase_admin │ public │ function │ postgres=X/supabase_admin ↵│
_33
│ │ │ │ anon=X/supabase_admin ↵│
_33
│ │ │ │ authenticated=X/supabase_admin ↵│
_33
│ │ │ │ service_role=X/supabase_admin │
_33
│ supabase_admin │ public │ sequence │ postgres=rwU/supabase_admin ↵│
_33
│ │ │ │ anon=rwU/supabase_admin ↵│
_33
│ │ │ │ authenticated=rwU/supabase_admin ↵│
_33
│ │ │ │ service_role=rwU/supabase_admin │
_33
│ supabase_admin │ public │ table │ postgres=arwdDxt/supabase_admin ↵│
_33
│ │ │ │ anon=arwdDxt/supabase_admin ↵│
_33
│ │ │ │ authenticated=arwdDxt/supabase_admin↵│
_33
│ │ │ │ service_role=arwdDxt/supabase_admin │
_33
└────────────────┴────────┴──────────┴──────────────────────────────────────┘
_33
(7 rows)

The first line now indicates the default access privilege we just added. Let’s now create a new table and insert a row in it:


_10
# as junior_dev
_10
postgres=> create table public.users(id serial primary key, name text);
_10
CREATE TABLE
_10
postgres=> insert into public.users(name) values ('john doe');
_10
INSERT 0 1

Now try to select data in public.users from senior_dev:


_10
# as senior_dev
_10
postgres=> select * from public.users;
_10
┌────┬──────────┐
_10
│ id │ name │
_10
├────┼──────────┤
_10
│ 1 │ john doe │
_10
└────┴──────────┘
_10
(1 row)

Note that we were immediately able to select data from public.users without explicit grants from junior_dev.

It is clear from above that the owner has all the privileges on an object which they can grant to other roles. But it can become cumbersome for the owner to keep granting the same privileges to every new role. There is a better way. We can ensure that objects are owned by a group and then any users which need access to those objects are assigned membership to the group. Let’s see how this works.

Creating Groups

We want to create a new developers group which will own the public.apps table. Then we will make junior_dev and senior_dev members of the developers group. This will ensure that they both have the same kind of access, without explicitly granting privileges after creating a new object.

First, let’s drop the public.apps table:


_10
# as junior_dev
_10
postgres=> drop table public.apps;
_10
DROP TABLE

Let’s also revoke the create privilege from junior_dev on the public schema:


_10
# as postgres
_10
postgres=> revoke create on schema public from junior_dev;
_10
REVOKE

Let’s create a developers group. Since a group is a role that is not allowed to login, use the nologin parameter:


_10
# as postgres
_10
postgres=> create role developers nologin;
_10
CREATE ROLE

You can't login with the developers role because we set the nologin parameter. The login/nologin parameters control the login attribute of a role. Earlier we also set the password attribute of the junior_dev and senior_dev roles. There are many other role attributes which we will talk about later in the post.

Let’s give the create privilege to the developers group:


_10
# as postgres
_10
postgres=> grant create on schema public to developers;
_10
GRANT

Since junior_dev and senior_dev users do not have create privilege on the public schema, they can’t create objects in it. The developers group can, but we can’t login with it. So how do we create public.apps owned by developers? Well, a user can temporarily impersonate a group if they are a member of the group. So let’s ensure junior_dev and senior_dev are members of the developers group:


_10
# as postgres
_10
postgres=> grant developers to junior_dev;
_10
GRANT ROLE
_10
postgres=> grant developers to senior_dev;
_10
GRANT ROLE

The grant <group> to <user> is another variant of the grant command but should be mentally read as add <user> to <group>.

Now junior_dev (or senior_dev) can impersonate developers:


_10
# as junior_dev
_10
postgres=> set role developers;
_10
SET
_10
postgres=> select current_role;
_10
┌──────────────┐
_10
│ current_role │
_10
├──────────────┤
_10
│ developers │
_10
└──────────────┘
_10
(1 row)

And create the public.apps table:


_10
# as junior_dev
_10
postgres=> create table public.apps(id serial primary key, name text);
_10
CREATE TABLE

Which is owned by the developers group:


_10
# as junior_dev
_10
postgres=> \dt public.apps
_10
List of relations
_10
┌────────┬──────┬───────┬────────────┐
_10
│ Schema │ Name │ Type │ Owner │
_10
├────────┼──────┼───────┼────────────┤
_10
│ public │ apps │ table │ developers │
_10
└────────┴──────┴───────┴────────────┘
_10
(1 row)

Now if you stop impersonation:


_10
# as junior_dev
_10
postgres=> reset role;
_10
RESET
_10
postgres=> select current_role;
_10
┌──────────────┐
_10
│ current_role │
_10
├──────────────┤
_10
│ junior_dev │
_10
└──────────────┘
_10
(1 row)

And try to insert or select data from public.apps it works:


_10
# as junior_dev or senior_dev
_10
postgres=> insert into public.apps(name) values ('next app');
_10
INSERT 0 1
_10
postgres=> select * from public.apps;
_10
┌────┬──────────┐
_10
│ id │ name │
_10
├────┼──────────┤
_10
│ 1 │ next app │
_10
└────┴──────────┘
_10
(1 row)

The reason junior_dev and senior_dev are able to insert and select data is because they are part of the developers group. If a new developer is created later, they are just a grant developers to <new dev> away from having the same access as every other developer. Contrast this with the previous method in which the new user would have to ask the owner of every object to grant them permissions.

Grant Options

Making a user part of another group might grant it three abilities:

  1. The ability to impersonate the group.
  2. The ability to inherit the permissions from the group.
  3. The ability to add or remove other users from the group.

All of these abilities can be controlled independently while running the grant <group> to <user> command by using the with <option name> true/false suffixed to it. The names of each of the above options are set, inherit, and admin. For example, to disallow a user from impersonating a group run grant <group> to <user> with set false.

To demonstrate, if we enable admin option on junior_dev:


_10
# as postgres
_10
postgres=> grant developers to junior_dev with admin option;
_10
GRANT ROLE

It will be able to remove senior_dev from the developers group:


_10
# as junior_dev
_10
postgres=> revoke developers from senior_dev;
_10
REVOKE ROLE

Without the admin option, junior_dev wouldn’t have been able to do this.

Role Attributes

Every role has some attributes associated with it which control the behavior of the role. Some of the common ones are listed below. For the full list and their details, refer to the Postgres role attributes documentation.

  • login - controls the role’s ability to login.
  • superuser - controls whether the role is a superuser or not. See next section for details.
  • createdb - controls whether the role will be able to create databases.
  • createrole - controls whether the role will be able to create other roles.
  • replication - controls whether the role can be used to initiate replication.
  • bypassrls - controls whether the role can bypass row level security.
  • connection limit - limits the maximum number of connections that the role can make to the database.
  • inherit - controls whether the role can inherit permissions from roles it is a member of.

Special Roles

There are two special roles which play an important part in how roles and privileges are managed.

Superuser

A superuser is a role with the superuser attribute set. A superuser is like a root user on the *nix OSes. It is very powerful and bypasses all privilege checks except authentication during login. For this reason, you should avoid working with this role as much as possible. Only superusers can create other superuser roles.

Public

public is a group role which every other role is automatically a part of. There is only one public role. So unlike superuser, there’s no public role attribute. The public role is used to provide privileges which are considered to be so common that every role should have them. These privileges are:

  • connect - ability to connect to the database.
  • temporary - ability to create temporary tables.
  • execute - ability to execute functions.
  • usage - ability to use an object like a domain, language or type.

The public role can’t be deleted, but its privileges can be revoked.

Privileges of a role are union of three sets of privileges:

  1. Those granted to the role directly.
  2. Those inherited from the roles this role is an explicit member of.
  3. Those inherited from the public role, which every role is implicitly a member of.

Privileges inherited from the public role are a common source of confusion when working with roles in Postgres. Imagine that we want to disallow junior_dev from executing functions. Let’s first create a function:


_10
# as postgres
_10
postgres=> create function add(integer, integer)
_10
returns integer
_10
as 'select $1 + $2;'
_10
language sql;
_10
CREATE FUNCTION

junior_dev is currently able to execute this function:


_10
# as junior_dev
_10
postgres=> select add(1, 2);
_10
┌─────┐
_10
│ add │
_10
├─────┤
_10
│ 3 │
_10
└─────┘
_10
(1 row)

Now let’s revoke junior_dev's execute permission:


_10
# as postgres
_10
postgres=> revoke execute on function add(integer, integer) from junior_dev;
_10
REVOKE

But junior_dev is still able to execute the function:


_10
# as junior_dev
_10
postgres=> select add(1, 2);
_10
┌─────┐
_10
│ add │
_10
├─────┤
_10
│ 3 │
_10
└─────┘
_10
(1 row)

How? Let’s check add function’s privileges:


_12
# as postgres
_12
postgres=> \df+ add
_12
┌────────┬──────┬──────────────────┬─────────────────────┬──────┬────────────┬──────────┬──────────┬──────────┬──────────────────────────┬──────────┐
_12
│ Schema │ Name │ Result data type │ Argument data types │ Type │ Volatility │ Parallel │ Owner │ Security │ Access privileges │ Language │
_12
├────────┼──────┼──────────────────┼─────────────────────┼──────┼────────────┼──────────┼──────────┼──────────┼──────────────────────────┼──────────│
_12
│ public │ add │ integer │ integer, integer │ func │ volatile │ unsafe │ postgres │ invoker │ =X/postgres ↵│ sql │
_12
│ │ │ │ │ │ │ │ │ │ postgres=X/postgres ↵│ │
_12
│ │ │ │ │ │ │ │ │ │ anon=X/postgres ↵│ │
_12
│ │ │ │ │ │ │ │ │ │ authenticated=X/postgres↵│ │
_12
│ │ │ │ │ │ │ │ │ │ service_role=X/postgres │ │
_12
└────────┴──────┴──────────────────┴─────────────────────┴──────┴────────────┴──────────┴──────────┴──────────┴──────────────────────────┴──────────┘
_12
(1 row)

junior_dev doesn’t have any privilege, but the missing role name in the =X/postgres line means the public role. Let’s revoke execute from public:


_14
# as postgres
_14
postgres=> revoke execute on function add(integer, integer) from public;
_14
REVOKE
_14
postgres=> \df+ add
_14
┌────────┬──────┬──────────────────┬─────────────────────┬──────┬────────────┬──────────┬──────────┬──────────┬──────────────────────────┬──────────┐
_14
│ Schema │ Name │ Result data type │ Argument data types │ Type │ Volatility │ Parallel │ Owner │ Security │ Access privileges │ Language │
_14
├────────┼──────┼──────────────────┼─────────────────────┼──────┼────────────┼──────────┼──────────┼──────────┼──────────────────────────┼──────────│
_14
│ public │ add │ integer │ integer, integer │ func │ volatile │ unsafe │ postgres │ invoker │ postgres=X/postgres ↵│ sql │
_14
│ │ │ │ │ │ │ │ │ │ anon=X/postgres ↵│ │
_14
│ │ │ │ │ │ │ │ │ │ authenticated=X/postgres↵│ │
_14
│ │ │ │ │ │ │ │ │ │ service_role=X/postgres │ │
_14
│ │ │ │ │ │ │ │ │ │ │ │
_14
└────────┴──────┴──────────────────┴─────────────────────┴──────┴────────────┴──────────┴──────────┴──────────┴──────────────────────────┴──────────┘
_14
(1 row)

Now junior_dev can not longer execute the add function:


_10
# as junior_dev
_10
postgres=> select add(1, 2);
_10
ERROR: permission denied for function add

Another thing to note here is that when we revoked execute privilege on add from junior_dev, there was actually nothing to revoke. But Postgres did not show us any warning. So it is important to always explicitly check the permissions, especially after a revoke command.

Summary

To summarize:

  • Every database object has an owner.
  • Operations on database objects are controlled by privileges.
  • Owners can grant privileges on owned objects to other roles.
  • Roles can be either users or groups.
  • Roles can inherit permissions from roles they are a member of.
  • public role is a role which every other role is implicitly a member of. It can’t be deleted, but its privileges can be revoked.
  • superuser roles are all powerful roles that bypass all privilege checks and should be used with care.
  • grant command only grants privileges on existing objects.
  • Default privileges control privileges to be granted to objects created in the future.

Conclusion

Postgres permissions follow the traditional objects, roles, privileges model but it has its subtleties which can surprise users unless they understand it in detail. In this post we experimented with this model to understand it in depth. Hope this understanding will allow you to manage and protect your Postgres database more effectively.

Share this article

Build in a weekend, scale to millions