ALTER DEFAULT PRIVILEGES - permission denied I have a problem with ALTER DEFAULT PRIVILEGES. I grant the permission:

ALTER DEFAULT PRIVILEGES 
    FOR USER user_name
    IN SCHEMA schema_name
    GRANT SELECT ON TABLES TO user_name;


in next step I create a table

create table schema_name.a (q int);


Now, I connect to dabasabe using conn_user and try to read the data in the new table. 

select * from schema_name.a


and I receive the error:

SQL Error [42501]: ERROR: permission denied for relation a
  org.postgresql.util.PSQLException: ERROR: permission denied for relation a


Do you have any ideas why the error appears?
The problem here is the ALTER DEFAULT PRIVILEGES statement you issued will apply only to tables that are created by user_name:

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

and

target_role
The name of an existing role of which the current role is a member. If FOR ROLE is omitted, the current role is assumed.

So in your case you either use
ALTER DEFAULT PRIVILEGES 
    IN SCHEMA schema_name
    GRANT SELECT ON TABLES TO user_name;

(without the FOR USER clause), or create the table as user_name (in which case they'll be the owner, therefore no need to grant anything).
In any case, it's a good practice to always create DB objects as a given user, so that the access privileges are predictable.