app.py

#!/usr/bin/env python3

import os
import psycopg2
from flask import (
    Flask,
    Response,
)

DATABASE_URL = os.environ.get('DATABASE_URL')

app = Flask(__name__)

@app.route('/')
def root():
    with psycopg2.connect(DATABASE_URL) as conn:
        with conn.cursor() as curs:
            curs.execute('SELECT * FROM testtable')
            result = curs.fetchall()

    return Response(result[0][0])

if __name__ == '__main__':
    app.run()

Procfile

web: gunicorn app:app --log-file=-

requirements.txt

gunicorn==20.1.0
Flask==2.0.2
psycopg2==2.9.3