CREATE OR REPLACE FUNCTION main_func (
  report_date timestamp, report_category INTEGER
)
RETURNS TABLE (
field 1 int, field2 int, field3 int)
) AS
$body$
BEGIN
IF report_category  = 1 THEN
select * from func1(report_date);
ELSE
select * from func2(report_date);
END IF;
END;
$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;

Select * from main_func ('2015-08-01', 1);

ERROR:  query has no destination for result data
HINT:  If you want to discard the results of a SELECT, use PERFORM instead.
CONTEXT:  PL/pgSQL function main_func (timestamp without time zone,integer) line 6 at SQL statement
(execution time: 375 ms; total time: 594 ms)

RETURNS TABLE (
field 1 int, field2 int, field3 int)
) AS

CREATE OR REPLACE FUNCTION main_func (
  report_date timestamp, report_category INTEGER
)
RETURNS TABLE (
field 1 int, field2 int, field3 int)
) AS
$body$
BEGIN
IF report_category  = 1 THEN
    RETURN QUERY (select * from func1(report_date));
ELSE
    RETURN QUERY (select * from func2(report_date));
END IF;
END;
$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;

CREATE OR REPLACE FUNCTION main_func (
  report_date timestamp, report_category INTEGER
)
RETURNS TABLE (
field 1 int, field2 int, field3 int)
) AS
$body$
BEGIN
IF report_category  = 1 THEN
    RETURN QUERY (select * from func1(report_date));
ELSE
    RETURN QUERY (select * from func2(report_date));
END IF;
END;
$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;

BEGIN
IF report_category  = 1 THEN
    RETURN QUERY (select * from func1(report_date));
ELSE
    RETURN QUERY (select * from func2(report_date));
END IF;
RETURN; --   <------- вот этот return мешал
END;