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

The following bug has been logged on the website:

Bug reference:      16395
Logged by:          Mario Leonel
Email address:      mario.ab.leonel@gmail.com
PostgreSQL version: 12.2
Operating system:   Ubuntu 18.04
Description:

-- 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

test=# SELECT id, field1, field2 FROM public.test;
 id | field1 | field2 
----+--------+--------
  1 |      2 | one
  2 |      4 | two
  3 |      6 | three
  4 |      8 | four
(4 rows)

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