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:

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:

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

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:

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

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:

Creating Objects and Assigning Privileges#

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

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:

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.

Let’s check the permissions again:

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

Let’s insert some data in it:

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

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>:

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:

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:

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:

Now senior_dev can select the data:

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:

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

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

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:

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

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:

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

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:

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

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:

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

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

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:

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:

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:

And create the public.apps table:

Which is owned by the developers group:

Now if you stop impersonation:

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

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:

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

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:

junior_dev is currently able to execute this function:

Now let’s revoke junior_dev's execute permission:

But junior_dev is still able to execute the function:

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

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:

Now junior_dev can not longer execute the add function:

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.