Create a recursive connections in PostgreSQL FDW, exhausting your client connections

recursive_fdw.sql

DO $d$
     BEGIN
         EXECUTE $$CREATE SERVER loopback FOREIGN DATA WRAPPER postgres_fdw
            OPTIONS (dbname '$$||current_database()||$$',
                     port '$$||current_setting('port')||$$'
            )$$;
     END;
$d$;

CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
CREATE TABLE foo ( id integer );
CREATE FOREIGN TABLE foo_fdw ( id integer ) server loopback;

SELECT * FROM foo_fdw;

The default target table in the CREATE FOREIGN TABLE statement is just the name of the foreign table itself (the code assumes the table will be remote) so this sets up a self reference. Each attempt to examine foo_fdw leads the backend to open a new connection to figure out what the structure of foo_fdw is. Even though each call only makes one new connection, all connections are eventually exhausted.