alter default privileges of certain column - sql

Question

I want a RoleGroup to have certain privileges on a specific column in a table ….the roleGroup not DB owner :
grant SELECT, INSERT ,UPDATE (colOne) on table schemaOne.tableOne
to roleGroup;
When I want alter default privileges of this column (to include future users), I get ERROR saying can’t alter default privileges of one column only :
alter default privileges for role DB_OWNER in schema schemaOne grant
select,insert,update(colOne) on table schemaOne.tableOne to
roleGroup;
which forced me to do this :
Alter Default priviliges for role DB_OWNER in schema schemOne grant
select,insert,update to roleGroup ;
what’s the point of the first grant then ?!! Or am I making some mistake ?

Answer 1

The whole question is a bit unclear as both of your statements are syntactically incorrect.
GRANT grants privileges on existing objects, while ALTER DEFAULT PRIVILEGES defines what privileges will automatically be granted to objects created in the future. So the scope of these statements does not overlap, but they complement each other.
I guess you misunderstood that point, otherwise you wouldn't try to ALTER DEFAULT PRIVILEGES on an existing table.
A valid form would be:
ALTER DEFAULT PRIVILEGES FOR ROLE db_owner IN SCHEMA schemaone
GRANT INSERT, SELECT, UPDATE ON TABLES TO rolegroup;
This will grant the privileges on all tables created in that schema by db_owner in the future.
The title of your question suggests that you are wondering why you cannot use ALTER DEFAULT PRIVILEGES to grant privileges on columns.
The likely answer is that not all future tables need to have a column colone, and what should happen with tables that don't have a column of that name. You might argue that no privileges should be granted in that case, and I guess that is a valid option. But it seems hard to come up with a use case for that, which may explain why nobody has considered implementing such a feature so far.
Alternatively, your desire could be that a column that will be added to an existing table in the future should automatically receive the privileges, but again I don't consider such a feature as very useful.