Postgresql function return multiple select statements - postgresql

Question

Can any one of you tell me how to approach this:
CREATE OR REPLACE FUNCTION name()
RETURNS ????? AS
$func$
BEGIN
SELECT * FROM tbl_a a;
SELECT * FROM tbl_b b;
END
$func$ LANGUAGE plpgsql;
Both tables have different structures.

Answer 1

You can use cursors but I can hardly imagine why you need such a function.
CREATE OR REPLACE FUNCTION my_multiselect(refcursor, refcursor) RETURNS VOID AS
$func$
BEGIN
OPEN $1 FOR SELECT * FROM information_schema.routines;
OPEN $2 FOR SELECT * FROM information_schema.sequences;
END
$func$ LANGUAGE plpgsql;
BEGIN;
SELECT my_multiselect('first_cursor_to_routines', 'second_cursor_to_sequences');
FETCH ALL IN first_cursor_to_routines;
FETCH ALL IN second_cursor_to_sequences;
COMMIT;

Answer 2

I'm not really sure what you're doing with this, but it sounds like you just want to return a union of these distinct result sets. You can do this with a dynamic query. I'm using Postgres 9.4.
CREATE OR REPLACE FUNCTION make_query(IN p_tables text[])
RETURNS void AS
$BODY$
DECLARE
v_qry text;
v_cols text;
v_types text;
v_as text;
BEGIN
EXECUTE format('
WITH sub AS (
SELECT
table_name,
column_name,
data_type
FROM
information_schema.columns
WHERE
table_name = ANY(%L)
ORDER BY
table_name,
ordinal_position)
,sub2 AS(
SELECT
DISTINCT ON (column_name, data_type)
column_name || '' '' || data_type AS def
FROM
sub
)
SELECT
string_agg(def, '','')
FROM
sub2;
',
p_tables
) INTO v_types;
v_qry := '
CREATE OR REPLACE FUNCTION name()
RETURNS TABLE(' || v_types || ') AS
$func$';
FOR i IN 1..array_upper(p_tables, 1)
LOOP
v_as := 'tbl' || i;
EXECUTE format('
WITH sub AS (
SELECT
table_name,
column_name,
data_type
FROM
information_schema.columns
WHERE
table_name = ANY(%L)
ORDER BY
table_name,
ordinal_position)
,sub2 AS(
SELECT
DISTINCT ON (column_name, data_type)
CASE WHEN table_name = ''%I''
THEN %L || ''.'' || column_name
ELSE ''NULL::'' || data_type
END AS cols
FROM
sub
)
SELECT
string_agg(cols, '','')
FROM
sub2;
',
p_tables,
p_tables[i],
v_as
) INTO v_cols;
IF i > 1 THEN
v_qry := v_qry || '
UNION ALL';
END IF;
v_qry := v_qry || '
SELECT ' || v_cols || ' FROM ' || p_tables[i] || ' AS ' || v_as;
IF i = array_upper(p_tables, 1) THEN
v_qry := v_qry || ';';
END IF;
END LOOP;
v_qry := v_qry || '
$func$ LANGUAGE sql;
';
EXECUTE v_qry;
END;
$BODY$
LANGUAGE plpgsql VOLATILE;
Sorry it looks ugly here, but this formatting helps the final product look nicer. If you're shy about executing a dynamic query like this off the bat, just replace EXECUTE v_qry; with RAISE INFO 'v_qry: %', v_qry; and it will simply print the dynamic query out in a message without executing it, so you can review what it will do once executed.
Then execute make_query() with a list of tables you want to display like this:
SELECT make_query(ARRAY['tbl_a', 'tbl_b']);
The result is that you will now have a function called name() which you can call in order to see the results of both tables at the same time, with all the union details already sorted out:
SELECT * FROM name();