CREATE OR REPLACE FUNCTION A()
  RETURNS VOID AS
$BODY$
                how do i print 'hello world' here?
 $BODY$
  LANGUAGE sql VOLATILE

CREATE OR REPLACE FUNCTION A()
  RETURNS VOID AS
$BODY$
        Raise Notice 'hello world'
 $BODY$
  LANGUAGE plpgsql VOLATILE

! echo "HERE YOUR DEBUG MSG!"
select * from your_table;

create function print(text) returns void as
$$ begin raise notice '%', $1; end
$$ language plpgsql;

create function whatever(int) returns int as $$
  select print('foobar');
  select $1;
 $$ language sql;

select whatever(3);
NOTICE:  foobar
CONTEXT:  SQL function "whatever" statement 1
 whatever
----------
        3
(1 row)

select 'Straight';
echo BETWEEN
select 'the lines.';

?column?
----------
 Straight
(1 row)

BETWEEN
  ?column?
------------
 the lines.
(1 row)