How can I UNION two tables in different PostgeSQL databases? - postgresql

Question

I'm running PostgreSQL 11.8 in a Docker container. I have two databases: website_db and testdb.
website_db has a products table with id, product_name, colour, product_size columns
testdb has a table called users with id, username, password
I'm using website_db and I want to UNION columns from the users table in the testdb database. I can get this to work in MySQL but am struggling with Postgres. Here's my attempt:
SELECT * FROM products WHERE product_name = 'doesntexist' OR 1=1 UNION SELECT null,username,password,null FROM testdb.users;
I get this error back:
ERROR: relation "testdb.users" does not exist
LINE 1: ...1=1 UNION SELECT null,username,password,null FROM testdb.use...
Does anyone know what I have to do to fix my query?

Answer 1

You can do it using dblink:
create database first;
create database second;
\c first;
create table products
(
id serial not null
constraint products_pk
primary key,
product_name varchar(50) not null
);
INSERT INTO public.products (id, product_name) VALUES (1, 'first_db');
\c second;
create table products
(
id serial not null
constraint products_pk
primary key,
product_name varchar(50) not null
);
INSERT INTO public.products (id, product_name) VALUES (1, 'sec_db');
-- dblink -- executes a query in a remote database
create extension dblink;
-- change queries and creds
SELECT id, product_name FROM products
UNION ALL
SELECT * FROM dblink('dbname=first user=root password=root', 'SELECT id, product_name FROM products') AS tb2(id int, product_name text);