Postgres' SET ROLE usage example

create group G1;
create group G2;

create user U1 password 'u1';

alter group G1 add user U1;
alter group G2 add user U1;

create table T1( a int );
create table T2( a int );

grant select on T1 to G1;
grant select on T2 to G2;

\c - u1

select * from t1;
select * from t2;

set role g1;

/* Should throw error, since we have explicitly taken on the garb of G1, hence giving up permissions of group G2 */

select * from t2;