PostgreSQL 9.5 - decode / select case to solve error with utf8 doesn't work

In continuation of my last post - "migration oracle to postgresql invalid byte sequence for encoding “UTF8”: 0x00"

I'm trying to insert into a local PostgreSQL table data from remote Oracle table (via oracle_fdw extension). My Oracle table has a column named street and it has valid string values and sometimes the next invalid (in PostgreSQL) string : ' ' (space).

When I try to copy the column value I get the error I mentioned above and in my last post. I understood that I need to change the oracle data before I insert it to PostgreSQL. I must do it on the fly so I tried to search for oracle decode func in PostgreSQL. I found two solution and I used both of them but I got same error:

1.using select with case :

mydb=>select *,(case when v.street=' ' then null END) from customer_prod v;
ERROR:  invalid byte sequence for encoding "UTF8": 0x00
CONTEXT:  converting column "street" for foreign table scan of 
 "customer_prod", row 254148

---

2.using decode function from orafce extension :

mydb=>select decode(street,' ',null) from customer_prod;
ERROR:  invalid byte sequence for encoding "UTF8": 0x00

So, I'm still getting the error. How can I solve this issue?

The error occurs when the values are transferred from Oracle to PostgreSQL, so post-processing won't prevent the error.

For the sake of demonstration, let's create an Oracle table that exhibits the problem:

CREATE TABLE nulltest(
   id number(5) CONSTRAINT nulltest_pkey PRIMARY KEY,
   val varchar2(10 CHAR)
);

INSERT INTO nulltest VALUES (1, 'schön');
INSERT INTO nulltest VALUES (2, 'bö' || CHR(0) || 'se');
INSERT INTO nulltest VALUES (3, 'egal');

COMMIT;

Let's create a foreign table in PostgreSQL for it:

CREATE FOREIGN TABLE nulltest (
   id integer OPTIONS (key 'true') NOT NULL,
   val varchar(10)
) SERVER oracle
   OPTIONS (table 'NULLTEST');

SELECT * FROM nulltest;

ERROR:  invalid byte sequence for encoding "UTF8": 0x00
CONTEXT:  converting column "val" for foreign table scan of "nulltest", row 2

Now the easiest thing would be to create a foreign table that filters away the zero characters:

CREATE FOREIGN TABLE filter_nulltest (
   id integer OPTIONS (key 'true') NOT NULL,
   val varchar(10)
) SERVER oracle
   OPTIONS (table '(SELECT id, replace(val, CHR(0), NULL) FROM nulltest)');

SELECT * FROM filter_nulltest;

┌────┬───────┐
│ id │  val  │
├────┼───────┤
│  1 │ schön │
│  2 │ böse  │
│  3 │ egal  │
└────┴───────┘
(3 rows)

Another, less efficient, option would be to create a function that catches and reports bad lines to you so that you can fix them on the Oracle side:

CREATE OR REPLACE FUNCTION get_nulltest() RETURNS SETOF nulltest
   LANGUAGE plpgsql AS
$$DECLARE
   v_id integer;
   n nulltest;
BEGIN
   FOR v_id IN SELECT id FROM nulltest
   LOOP
      BEGIN
         SELECT nulltest.* INTO n
            FROM nulltest
            WHERE id = v_id;
         RETURN NEXT n;
      EXCEPTION
         WHEN OTHERS THEN
            RAISE NOTICE 'Caught error % for id=%: %', SQLSTATE, v_id, SQLERRM;
      END;
   END LOOP;
END;$$;

SELECT * FROM get_nulltest();

NOTICE:  Caught error 22021 for id=2: invalid byte sequence for encoding "UTF8": 0x00
┌────┬───────┐
│ id │  val  │
├────┼───────┤
│  1 │ schön │
│  3 │ egal  │
└────┴───────┘
(2 rows)