[DNM] postgres_fdw arbitrary queries

[rafatower]

A PoC of a function to execute arbitrary queries through a foreign server.

Signature:

CREATE FUNCTION postgres_fdw_query(server name, sql text)
RETURNS SETOF record

Examples of usage:

SELECT * FROM postgres_fdw_query('cdb_fdw_local_pg11', 'SELECT nspname, nspowner from pg_namespace') AS schemas(schema_name name, owner oid);
    schema_name     | owner 
--------------------+-------
 pg_toast           |    10
 pg_temp_1          |    10
 pg_toast_temp_1    |    10
 pg_catalog         |    10
 public             |    10
 information_schema |    10
 carto_lite         | 16384
(7 rows)

SELECT * FROM postgres_fdw_query('cdb_fdw_local_pg11', 'SELECT 1') ;
ERROR:  a column definition list is required for functions returning "record"
LINE 1: SELECT * FROM postgres_fdw_query('cdb_fdw_local_pg11', 'SELE...

SELECT * FROM postgres_fdw_query('cdb_fdw_local_pg11', 'SELECT 1') AS t1(i int);
 i 
---
 1
(1 row)