postgres=# drop table if exists tt;
NOTICE:  table "tt" does not exist, skipping
DROP TABLE
postgres=# create temp  table tt (name varchar, id int);
CREATE TABLE
postgres=# insert into tt values (1,1), (2,2);
INSERT 0 2
postgres=#  select json_agg(tt) from tt; 
        json_agg        
------------------------
 [{"name":"1","id":1}, +
  {"name":"2","id":2}]
(1 row)

drop table if exists tt;
create temp  table tt (name varchar, id int, f1 varchar, f2 varchar);
insert into tt values (1,1), (2,2);
 select json_agg(foo) 
  from (select name, id 
          from tt) as foo;