create extension IF NOT EXISTS dblink;
-- без предустановки соединения:
SELECT * FROM dblink('dbname=test password=postgres','SELECT 1;') AS 
	t (f integer);
-- 1

-- проверка, что соединение, в таком случае, не держится:
SELECT * FROM dblink('SELECT 1;') AS t (f integer);
-- ERROR: connection not available
-- SQL-состояние: 08003
-- открытие явного неименованного соединения
SELECT dblink_connect('dbname=test password=postgres');
-- OK
-- проверка, что соединение, в таком случае, держится:
SELECT * FROM dblink('SELECT 1;') AS t (f integer);
-- 1
SELECT * FROM dblink('SELECT 1;') AS t (f integer);
-- 1
-- закрытие неименованного соединения:
SELECT dblink_disconnect();
--'OK'
SELECT * FROM dblink('SELECT 1;') AS t (f integer);
--ERROR: connection not available
--SQL-состояние: 08003

-- открытие именованного соединения 
SELECT dblink_connect('test connection','dbname=test password=postgres');
-- OK
SELECT dblink_connect('test connection','dbname=test password=postgres');
--ERROR: duplicate connection name
--SQL-состояние: 42710
-- попытка  использо как неименое
SELECT * FROM dblink('SELECT 1;') AS t (f integer);
--ERROR: connection not available
--SQL-состояние: 08003
SELECT * FROM dblink('test connection','SELECT 1;') AS t (f integer);
-- 1
SELECT * FROM dblink('test connection','SELECT version();') AS t (f text);
--'PostgreSQL 9.3.5 on x86_64-unknown-linux-gnu, compiled by gcc...
--
SELECT dblink_get_connections();
-- '{"test connection"}'
SELECT * FROM dblink('test connection','SELECT dblink_get_connections();') AS t (f text);
--RROR: function dblink_get_connections() does not exist
--SQL-состояние: 42883
SELECT * FROM dblink('test connection','create extension IF NOT EXISTS dblink;SELECT dblink_get_connections();') AS t (f text);
--'OK'
SELECT * FROM dblink('test connection','SELECT dblink_connect(''test connection'',''dbname='||current_database() ||' password=postgres'');') AS t (f text);
--'OK'
SELECT * FROM dblink('test connection','SELECT dblink_get_connections();') AS t (f text);
-- '{"test connection"}'
SELECT * FROM dblink('test connection','SELECT * FROM dblink(''test connection'', ''SELECT current_database()'') AS t (f text) ;') AS t (f text);
--'testdb'
SELECT * FROM dblink('test connection','SELECT * FROM dblink($$test connection$$,$x$SELECT dblink_connect($$test connection$$,$$dbname=test password=postgres$$);$x$) AS t (f text)') AS t (f text);
--'OK'
SELECT * FROM dblink('test connection','SELECT * FROM dblink($$test connection$$,$x$SELECT dblink_get_connections();$x$) AS t (f text)') AS t (f text);

SELECT current_database(), dblink_get_connections(),pg_backend_pid();
-- 'testdb','{"test connection"}',3558
SELECT * FROM dblink('test connection','SELECT * FROM dblink($$test connection$$,$x$SELECT current_database(), dblink_get_connections(),pg_backend_pid();;$x$)
	AS t (f text, f2 text[],f3 integer)')
	AS t (f text, f2 text[],f3 integer);
-- 'testdb','{"test connection"}',3707
--Внимание, впорос: почему  в одной БД 2 одноименных соединения  разных сеансах [PID] ?