Why can't PostgreSQL make this simple FULL JOIN?

Question

Here's a minimal setup with two tables aand b, each with three rows:

CREATE TABLE a (
    id SERIAL PRIMARY KEY,
    value TEXT
);
CREATE INDEX ON a (value);

CREATE TABLE b (
    id SERIAL PRIMARY KEY,
    value TEXT
);
CREATE INDEX ON b (value);

INSERT INTO a (value) VALUES ('x'), ('y'),        (NULL);
INSERT INTO b (value) VALUES        ('y'), ('z'), (NULL);

Here is a LEFT JOIN that works fine as expected:

SELECT * FROM a
LEFT JOIN b ON a.value IS NOT DISTINCT FROM b.value;

with output:

 id | value | id | value 
----+-------+----+-------
  1 | x     |    | 
  2 | y     |  1 | y
  3 |       |  3 | 
(3 rows)

Changing "LEFT JOIN" to "FULL JOIN" gives an error:

SELECT * FROM a
FULL JOIN b ON a.value IS NOT DISTINCT FROM b.value;

ERROR: FULL JOIN is supported only with merge or hash join conditions.

Can someone answer:

What is a "merge or hash join join condition" and why does the join in a.value IS NOT DISTINCT FROM b.valuenot fulfill this condition, but it a.value = b.valueworks great?

, , NULL. value , EXPLAIN NULL , , NULL:

EXPLAIN SELECT * FROM a WHERE value = 'x';
                                QUERY PLAN                                
--------------------------------------------------------------------------
 Bitmap Heap Scan on a  (cost=4.20..13.67 rows=6 width=36)
   Recheck Cond: (value = 'x'::text)
   ->  Bitmap Index Scan on a_value_idx  (cost=0.00..4.20 rows=6 width=0)
         Index Cond: (value = 'x'::text)


EXPLAIN SELECT * FROM a WHERE value ISNULL;
                                QUERY PLAN                                
--------------------------------------------------------------------------
 Bitmap Heap Scan on a  (cost=4.20..13.65 rows=6 width=36)
   Recheck Cond: (value IS NULL)
   ->  Bitmap Index Scan on a_value_idx  (cost=0.00..4.20 rows=6 width=0)
         Index Cond: (value IS NULL)

PostgreSQL 9.6.3 10beta1.

, .

Answer 1

PostgreSQL FULL OUTER JOIN , .

,

<expression using only left table> <operator> <expression using only right table>

, PostgreSQL IS NOT DISTINCT FROM, :

(NOT ($1 IS DISTINCT FROM $2))

- , .

:

SELECT a_id, NULLIF(a_value, '<null>'),
       b_id, NULLIF(b_value, '<null>')
FROM (SELECT id AS a_id,
             COALESCE(value, '<null>') AS a_value
      FROM a
     ) x
   FULL JOIN
     (SELECT id AS b_id,
             COALESCE(value, '<null>') AS b_value
      FROM b
     ) y
      ON x.a_value = y.b_value;

, <null> value.

Answer 2

, ON "TRUE" "ON" WHERE. , .