teledb=# create table teledb_pg1(id int, nickname varchar);
CREATE TABLE
teledb=# insert into teledb_pg1 values(3, 'pg'),(5,'test');
COPY 2
teledb=# select * from teledb_pg union all select * from teledb_pg1;
 id |  nickname  
----+------------
  1 | teledb
  1 | hello,pgxc
  2 | TELEDB
  3 | pg
  4 | 
  5 | test
  3 | pg
(7 rows)

teledb=# select * from teledb_pg union select * from teledb_pg1;
 id |  nickname  
----+------------
  1 | teledb
  1 | hello,pgxc
  4 | 
  5 | test
  2 | TELEDB
  3 | pg
(6 rows)

teledb=# select * from (select * from teledb_pg limit 2) as t union all select * from (select * from teledb_pg1 limit 2)
teledb-# ;
 id |  nickname  
----+------------
  1 | teledb
  1 | hello,pgxc
  5 | test
  3 | pg
(4 rows)