Why is RETURN QUERY returning a string instead of a TABLE - sql

Question

This MWE is NOT how you would typically solve this problem, however, it is as simple as I can explain the problem I am encountering. I am merely trying to point out 2 things
I am doing more than simply returning the contents of a Table
What is being returned is NOT being returned as a Table but a String
Supporting SQL Statements:
DROP DATABASE IF EXISTS test;
CREATE DATABASE test;
\c test
CREATE TABLE credit_card(
id BIGSERIAL PRIMARY KEY,
balance BIGINT
);
Functions:
CREATE FUNCTION get_credit_card(
p_id BIGINT
)
RETURNS TABLE(
id BIGINT,
balance BIGINT
)
AS $$
DECLARE
BEGIN
RETURN QUERY
SELECT
credit_card.id,
credit_card.balance
FROM
credit_card
WHERE
credit_card.id = p_id;
END $$ LAnguage 'plpgsql';
CREATE FUNCTION pay_with_card(
p_id BIGINT,
p_amount BIGINT
)
RETURNS TABLE(
id BIGINT,
balance BIGINT
)
AS $$
DECLARE
v_balance BIGINT;
BEGIN
SELECT
credit_card.balance
FROM
credit_card
INTO
v_balance
WHERE
credit_card.id = p_id;
IF v_balance < p_amount
THEN
RETURN;
END IF;
UPDATE
credit_card
SET
balance = credit_card.balance - p_amount;
RETURN QUERY
SELECT get_credit_card (p_id);
END $$ LAnguage 'plpgsql';
Populate Table and Call function:
INSERT INTO credit_card
(balance)
VALUES
(100);
SELECT
pay_with_card (1, 100);
Error:
DROP DATABASE
CREATE DATABASE
You are now connected to database "test" as user "postgres".
CREATE TABLE
CREATE FUNCTION
CREATE FUNCTION
INSERT 0 1
psql:test.sql:74: ERROR: structure of query does not match function result type
DETAIL: Returned type record does not match expected type bigint in column 1.
CONTEXT: PL/pgSQL function pay_with_card(bigint,bigint) line 24 at RETURN QUERY
It took me a long time to figure out that pay_with_card is returning a String, or what appears to be a String, instead of a TABLE(id BIGINT, balance BIGINT). With the Python psycopg2 library, the returned query is
[('(1,100)'),]
So my entire code is breaking because I can't get the values (unless I hack it and use string manipulation.
Question:
How can I fix it so that it returns the correct query like so
[(1,100),]

Answer 1

An alternative to the hint in horse_with_no_name's comment, you can replace
RETURN QUERY
SELECT get_credit_card (p_id);
with
RETURN QUERY SELECT (get_credit_card(p_id)).*;
You need some way of expanding the returned record back into its constituent fields. (I think horse’s SELECT * … has the same effect.)