Minimum privileges required for a user

The best way, really, is for the web app to have no right to create or modify tables at all. It should only have the rights to do what it needs to do to perform its day to day functions, not administration.

In particular, the web app must not run as a superuser and should preferably not own the database or tables.

If the webapp has built-in administration functions for changing tables when it's upgraded, etc, it should do so via a separate set of credentials and allow you to be prompted to enter those credentials each time you perform an upgrade. Or let you create the file with the admin credentials, run the upgrade, and delete it again.

If you can't fix the webapp to use different credentials for schema changes, then it might as well be the owner of the database and tables. Just don't make it a superuser!

If you can fix the app so it uses different credentials for schema changes, then its regular non-schema-change user should not own the tables or database. It should have only CONNECT and TEMPORARY rights on the database, USAGE rights on the schema(s), and the required rights on each table GRANTed to it. Just don't forget to REVOKE ALL FROM public ON DATABASE mydb; and REVOKE ALL FROM public ON SCHEMA public;.

The documentation about ALTER DEFAULT PRIVILEGES tells you a possible reason - it is not that clearly described, though.

Let's see, what is said:

You can change default privileges only for objects that will be created by yourself or by roles that you are a member of.

This means that the default privileges defined by this statement applies only to objects that are created by you (or the role you are a member of). Let's see this in action!

alice's schema

First, we are logged in as alice. Then, in a newly created schema, we create a table and grant some rights to bob:

SELECT current_user;
 current_user 
──────────────
 alice


SHOW search_path ;
 search_path  
──────────────
 test, public

CREATE SCHEMA alicetest;

ALTER DEFAULT PRIVILEGES 
    FOR ROLE alice 
    IN SCHEMA alicetest 
    GRANT ALL ON TABLES TO alice;

GRANT SELECT ON alicetest.a TO bob; 
-- this I do only for showing the privileges - 
-- the owner has by default ALL and is not shown by \dp

\dp alicetest.a
                             Access privileges
  Schema   │ Name │ Type  │  Access privileges  │ Column access privileges 
───────────┼──────┼───────┼─────────────────────┼──────────────────────────
 alicetest │ a    │ table │ alice=arwdDxt/alice↵│ 
           │      │       │ bob=r/alice         │ 

alice now has all rights on her table, as expected.

bob' table in the same schema

Now, after obtaining access to this schema, bob tries to create a table:

SELECT current_user;
 current_user 
──────────────
 bob

CREATE TABLE alicetest.b (id integer);

GRANT SELECT ON alicetest.b TO alice;

\dp alicetest.b
                            Access privileges
  Schema   │ Name │ Type  │ Access privileges │ Column access privileges 
───────────┼──────┼───────┼───────────────────┼──────────────────────────
 alicetest │ b    │ table │ bob=arwdDxt/bob  ↵│ 
           │      │       │ alice=r/bob       │ 

As you can see, despite creating the table in alice's schema where she set the default privileges, bob's table doesn't have all those permissions. This happens because alice is not a member of bob.

Let's check this membership thing, too, and try to define default privileges by alice again, this time for another role:

ALTER DEFAULT PRIVILEGES 
    FOR ROLE charlie 
    IN SCHEMA alicetest 
    GRANT ALL ON TABLES TO alice;
ERROR:  must be member of role "charlie"

So, some mighty enough user grants her a membership in charlie, then she tries again, with success:

ALTER DEFAULT PRIVILEGES 
    FOR ROLE charlie 
    IN SCHEMA alicetest 
    GRANT ALL ON TABLES TO alice;
ALTER DEFAULT PRIVILEGES

charlie's round

Then charlie creates a new table:

CREATE TABLE alicetest.c (id integer);

And the privileges:

\dp alicetest.c
                               Access privileges
  Schema   │ Name │ Type  │    Access privileges    │ Column access privileges 
───────────┼──────┼───────┼─────────────────────────┼──────────────────────────
 alicetest │ c    │ table │ charlie=arwdDxt/charlie↵│ 
           │      │       │ alice=arwdDxt/charlie   │ 

As you see, alice, as a member of charlie, gets her access to this table.

To answer your question,

I guess you defined the default privileges for yourself (alice in the example), but you developers act as a bunch of bobs here, not getting the necessary privileges. One way to get around this (as we do it at work) is to do a

SET ROLE TO schema_owner;

every time before creating a new object in the schema. This should be a role that all developers are a member of (otherwise you'd get an error).

NOTE that \dp is a psql command.