Save output from multiple SQL SELECT commands to a file
Basic function to return rows from multiple SELECT statements:

CREATE OR REPLACE FUNCTION f_get_rows()
  RETURNS TABLE(a_id integer, txt text) AS
$func$
BEGIN

RETURN QUERY SELECT a.a_id, a.txt FROM tbl_a a;

RETURN QUERY SELECT b.a_id, b.txt FROM tbl_b b;

END
$func$ LANGUAGE plpgsql;


You could do the same with UNION ALL.

Simple &amp; quick demo to output from tables with different structure:

CREATE OR REPLACE FUNCTION f_get_rows()
  RETURNS SETOF text AS
$func$
BEGIN

RETURN QUERY SELECT t::text FROM tbl_a t;

RETURN QUERY SELECT t::text FROM tbl_c t;

END
$func$ LANGUAGE plpgsql;


You can cast any type to text in PostgreSQL, even the composite type of a table row ...

Call:

SELECT * FROM f_get_rows()


To write this to a file (local to the server):

COPY (SELECT * FROM f_get_rows()) TO '/path/to/my/file.csv';


To get you started read these chapters in the manual:


CREATE FUNCTION
Returning From a Function
COPY


For a more specific answer you need to write a more specific question.