Also, the junction table approach still doesn't give me what I want, since th...

[Comment from a deleted post]

Also, the junction table approach still doesn't give me what I want, since the un-junctioned records collide on the join criteria:

[local] dian#dian= select * from left_table;
 id │  val  
────┼───────
  1 │ one
  2 │ two
  3 │ three
(3 rows)

Time: 0.559 ms
[local] dian#dian= select * from right_table;
 id │ val 
────┼─────
  1 │ 1
  2 │ 2
  3 │ 3
(3 rows)

Time: 0.587 ms
[local] dian#dian= select * from left_right;
 left_id │ right_id 
─────────┼──────────
       1 │        3
       3 │        1
(2 rows)

Time: 0.533 ms
[local] dian#dian= select * from left_table l
left outer join left_right lr on lr.left_id = l.id
right outer join right_table r on r.id = lr.right_id;
   id   │  val   │ left_id │ right_id │ id │ val 
────────┼────────┼─────────┼──────────┼────┼─────
      1 │ one    │       1 │        3 │  3 │ 3
      3 │ three  │       3 │        1 │  1 │ 1
 (null) │ (null) │  (null) │   (null) │  2 │ 2
(3 rows)

Time: 0.984 ms

To get unaffiliated lefts and rights to show up, you have to run the query twice and union the results:

[local] dian#dian= select * from left_table l
dian#dian- left outer join left_right lr on lr.left_id = l.id
dian#dian- left outer join right_table r on r.id = lr.right_id
dian#dian- union
dian#dian- select * from left_table l
dian#dian- left outer join left_right lr on lr.left_id = l.id
dian#dian- right outer join right_table r on r.id = lr.right_id;
   id   │  val   │ left_id │ right_id │   id   │  val   
────────┼────────┼─────────┼──────────┼────────┼────────
      3 │ three  │       3 │        1 │      1 │ 1
 (null) │ (null) │  (null) │   (null) │      2 │ 2
      2 │ two    │  (null) │   (null) │ (null) │ (null)
      1 │ one    │       1 │        3 │      3 │ 3
(4 rows)

Time: 0.697 ms