BUG #16395: error when selecting generated column in a foreign table

On 2020-04-27 16:24, PG Bug reporting form wrote:

-- bd: postgres
CREATE TABLE public.test
(
id integer NOT NULL,
field1 integer NOT NULL,
field2 character varying COLLATE pg_catalog."default",
CONSTRAINT test_pkey PRIMARY KEY (id)
);

-- bd: test
CREATE FOREIGN TABLE public.test(
id integer NULL,
field1 integer NULL,
field2 character varying NULL COLLATE pg_catalog."default",
field_calc integer generated always as (field1 * 2) stored
)
SERVER fs_postgres;

test=# SELECT id, field1, field2, field_calc FROM public.test;
ERROR: column "field_calc" does not exist
CONTEXT: remote SQL command: SELECT id, field1, field2, field_calc FROM
public.test

Well, your public.test base table does not in fact have a "field_calc"
column. You need to add that.

Note that these are *stored* generated columns, so the value that is
generated by the foreign table on insert is meant to be stored somewhere
in the backing store of the foreign table.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Here is a test case that shows that it works:

create extension postgres_fdw;

create server fs_postgres foreign data wrapper postgres_fdw options
(dbname 'postgres');

create user mapping for current_user server fs_postgres;

CREATE TABLE public.test (
id integer PRIMARY KEY,
field1 integer NOT NULL,
field2 character varying,
field_calc integer
);

CREATE FOREIGN TABLE public.test_ft (
id integer,
field1 integer,
field2 character varying,
field_calc integer generated always as (field1 * 2) stored
)
SERVER fs_postgres OPTIONS(table_name 'test');

insert into test_ft (id, field1, field2) values (1, 2, 'one');

select * from test_ft;
id | field1 | field2 | field_calc
----+--------+--------+------------
1 | 2 | one | 4

select * from test;
id | field1 | field2 | field_calc
----+--------+--------+------------
1 | 2 | one | 4

In your case, were you adding columns to existing tables? Depending on
the order of actions, it might not work the way one might think.

Can you reproduce your problem from a blank database?

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services