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I agree with the sentiment that IAM in PostgreSQL is complex.

What makes it complex is that there are 3 layers of objects (Database, Schema, Tables) and also implicit grants given to DB object owners

To be able to select from a table you need:

* CONNECT on the Database

* USAGE on the Schema (Given implicitly to schema owner)

* SELECT on the Table (Given implicitly to table owner)

To see these privileges we need to understand acl entries of this format

`grantee=privilege-abbreviation[]/grantor:`

* Use \l+ to see privileges of Database

* Use \dn+ to see privileges of Schemas

* Use \dp+ to see privileges of Tables

Privileges are seen [here](https://www.postgresql.org/docs/current/ddl-priv.html)

e.g. in the following example user has been given all permissions by postgres role

`user=arwdDxt/postgres`

If the “grantee” column is empty for a given object, it means the object has default owner privileges (all privileges) or it can mean privileges to PUBLIC role (every role that exists)

`=r/postgres`

Also it's confusing when Public schema is used. You have CREATE permission on schema so when the tables are created with the same user you select data with and you have owner permissions out of the box.

I've seen funny things happen when developers think their GRANT is doing what ALTER DEFAULT PRIVILEGES does, and a day later break production because their user doesn't have access to the new tables.

What do you mean exactly?

If roles have INHERIT, then doing the following works, no?

* Role A creates table * GRANT A TO B; * ROLE B can read from table just like A can.

Also if Role A creates new table, Role B can read that too no?

Remove any "if" from your comment and it seems like you understood what would fail! There's more ways to make misunderstanding grant vs default privileges will create issues as well.

Then I would argue for trying to centrally ensure that all roles have INHERIT in your DBs. :thinking:

Doing ALTER DEFAULT PRIVILEGES could be another future footgun of it's own.

I think you probably don’t want the application user to inherit from the superuser you may use for migrations since some migrations require a superuser.

> To be able to select from a table you need

> * CONNECT

> * USAGE

> * SELECT

Isn't LOGIN (https://www.postgresql.org/docs/16/role-attributes.html) also needed?

  Only roles that have the LOGIN attribute can be used as the initial role name for a database connection

You can log in and then change your role (if you permissions) with SET ROLE NEW_USER;

We managed to kludge our way to defaulting to read only, then using set role to do writes if you need to.

Good point. Connecting to the Cluster is indeed actually a 4th layer.

There you do need user with LOGIN, valid password & SSL.

IMO, the permission management of the MS SQL Server is the best one around. It gets all the basics right, without any big issue. Other DBMS should copy it, or use it as a starting point if they want to go further (and there is plenty of stuff to improve on it).

TLDR, the container objects and the contained ones all share the same kind of permissions. Permissions of the container are applied to the contained unless explicitly changed.

So, if you grant select on the schema dbo to a, a will get select on all tables there. If you want to remove some table, you revoke the select on that specific table. And there is both metadata to discover where a specific privilege comes from and specific commands that edit the privileges on a specific level.

> What makes it complex is that there are 3 layers of objects (Database, Schema, Tables) and also implicit grants given to DB object owners

The main privileges systems includes Columns, as well as Databases/Schemas/Tables. You can SELECT from a table if you have been granted SELECT on the table, or if you have been granted it on the specific columns used in your query. ("A user may perform SELECT, INSERT, etc. on a column if they hold that privilege for either the specific column or its whole table. Granting the privilege at the table level and then revoking it for one column will not do what one might wish: the table-level grant is unaffected by a column-level operation." [1])

There's also a system of Row Security Policies [2].

[1]: https://www.postgresql.org/docs/current/sql-grant.html

[2]: https://www.postgresql.org/docs/current/ddl-rowsecurity.html

Even the docs of postgrest, which relies on roles for auth, seems to be not very detailed: https://postgrest.org/en/v12/explanations/db_authz.html

Interesting. If you are serious about writing a cookbook for postgres roles, and open something like a kickstarter, I'll be one of the first to pledge!

> Making it work feels like black magic.

Can confirm. Last year I implemented a simple postgREST server with rowlevel security. (The postgREST logs are really good. With cookbook and all)

The path there was somewhat difficult, but once it worked, it was truly magical. And the mechanisms involved were quite simple even.

I’d read that. Role management for me often involves a lot of guessing and too often roles end up with too many permissions because of that.

Hah, role management for us is "create a role for migrations, and a role to do db things, and enforce auth entirely in the web app"

I suspect we aren't alone

This is a fairly sane place to be in terms of bang for your buck. It's easy to find yourself in a place where authorization data and logic span multiple services and at that point having everything deeply siloed into Postgres might be a doozy. That being said, there are plenty of times that'll never be the case and you should try to lean on the abstractions that work best for you.

Roles are for services not for users. If you have a read-only Web api then it makes sense to use a read-only role regardless of which user is using it.

Everything in PostgreSQL is a role.

It's just named such that when a ROLE allows `login` it's considered a user

This is part of what many people find so confusing. In most systems “role” is a group (or something closely resembling a group), not a user. The weird terminology confuses beginners

It's a bit confusing and legacy.

All roles function like you would expect groups to function

A role that is not allowed to login is a `group`.

While the CREATE USER and CREATE GROUP commands still exist, they are simply aliases for CREATE ROLE.

Someone was feeling very clever when they came up with this idea.

It makes a lot of sense, especially since in the SQL standard, you can grant privileges to "users" or "roles."

Might as well simplify the mental model and make them the same.

To me, the comment you are replying to is saying that you should^ DIFFERENTIATE roles by service, not ‘end user’.

> Hah, role management for us is "create a role for migrations, and a role to do db things, and enforce auth entirely in the web app"

> I suspect we aren't alone

Honestly I'd be happy to spend the time learning the ins and outs of PostgreSQL IAM stuff, but there's two very good reasons why I won't use it:

1. Still need the "role"/user across other services, so I don't save anything by doing ACL inside the DB.

2. I've no idea how to temporarily drop privileges for a single transaction. Connecting as the correct user on each incoming HTTP request is way too slow.

> 2. I've no idea how to temporarily drop privileges for a single transaction. Connecting as the correct user on each incoming HTTP request is way too slow.

`SET ROLE`[1] changes the "the current user identifier of the current SQL session"; after running it "permissions checking for SQL commands is carried out as though the named role were the one that had logged in originally".

Whilst it changes the "current user" it doesn't change the current "session user", and this is what determines which roles you can switch to.

The docs also note that:

> SQL does not allow this command during a transaction; PostgreSQL does not make this restriction because there is no reason to.

[1]: https://www.postgresql.org/docs/16/sql-set-role.html

Here's your cookbook: https://stackoverflow.com/questions/78401179/how-do-postgres...

Oh it's really worrying that you post a link to a SO page as the recommended 'cookbook' for something that's referred to as a powerful feature. And both of the answers only have a single up vote each. Seems like there's a serious gap here.

You can help out by posting answer / upvoting good answer.

I don't think Stack Overflow is a good place for good documentation. I think the Postgres documentation should be better.

> I need to sit down with it for a month and write "cookbook". If one person uses it and goes to bed that night without crying them selves to sleep it will have been worth it.

Please do. I'd be happy to pay ~$20 for it.

What I don't understand is why databases don't use tables/views to implement their native permissions systems.

Create a table to hold rows of (db,schema,table,role,read,write) configured by the admin with INSERT/UPDATE/DELETE, then a view that applies inheritance behavior and can answer whether any user can access any given resource.

Literally what PostgreSQL does.

E.g. you can do manual selects from internal tables to see the same content as `\dt` command for example.

Oh, cool. Where's the schema?

I have seen what you have seen (did several "serious" things using postgrest).

edit: i mean yes, we need that cookbook real bad

I recently had to rename an app I was developing. That included the linux user account under which the service ran, and therefore the postgres user.

I don't remember all the details, but I had to also rename the postgres user and role, which seemed a simple thing to do. But for some reason renaming the user didn't include the permissions on the database. I was left with a very confusing state of working table access and denied record access. I decided to backup the data, dropp the database and do an import that didn't include any permissions.

That simple thing turned out a complete shit show and I blame Postgres for making something so simple so complex.

I never use system accounts for Postgres auth. It feels like coupling that won’t do anything good for most people most of the time, and will only bite you in the ass when you don’t expect it to.