PostgreSQL, how to query the database to retrieve the percentile elements of a dataset

A recursive CTE seems the way to go.
Assuming your path has no cycles. Else it needs more work to detect cycles. The array solution below can readily be adapted.

Test setup

Building on this simplified layout:

CREATE TABLE t1 (t1_id int, objid text);
INSERT INTO t1 VALUES
 (1,'aaa')
,(2,'bbb')
,(3,'ccc')
,(4,'ddd')
,(5,'eee')
,(6,'fff')
,(7,'ggg')
,(8,'hhh');

CREATE TABLE t2 (t2_id int, t1_id int, objid text);
INSERT INTO t2 VALUES
 (1,3,'aaa')
,(2,4,'aaa')
,(3,7,'hhh')
,(4,8,'ccc');

Two different solutions:

Solution with string as path

WITH RECURSIVE cte AS (
   SELECT t.t1_id AS start_id
        , t2.t2_id::text || '(t2)' || COALESCE(' ->' || t1.t1_id || '(t1)', '') AS path
        , t1.t1_id
   FROM   t1 t
   LEFT   JOIN t2 USING (t1_id)
   LEFT   JOIN t1 ON t1.objid = t2.objid

   UNION ALL
   SELECT c.start_id
        , c.path || ' ->' || t2.t2_id || '(t2)' || COALESCE(' ->' || t1.t1_id || '(t1)', '')
        , t1.t1_id
   FROM   cte c
   JOIN   t2      USING (t1_id)
   LEFT   JOIN t1 USING (objid)
   )
SELECT DISTINCT ON (start_id)
       start_id, path
FROM   cte
ORDER  BY start_id, path DESC;

Result:

start_id   path
1          
2          
3          1(t2) ->1(t1)
4          2(t2) ->1(t1)
5          
6          
7          3(t2) ->8(t1) ->4(t2) ->3(t1) ->1(t2) ->1(t1)
8          4(t2) ->3(t1) ->1(t2) ->1(t1)

Table names are redundant, obviously. I added them for good looks.

Solution with inverted array

Rightmost element is first t2_id, keep alternating from right to left.

WITH RECURSIVE cte AS (
   SELECT t.t1_id AS start_id, ARRAY[t1.t1_id, t2.t2_id] AS path
   FROM   t1 t
   LEFT   JOIN t2 USING (t1_id)
   LEFT   JOIN t1 ON t1.objid = t2.objid

   UNION ALL
   SELECT c.start_id, t1.t1_id || (t2.t2_id || path)
   FROM   cte c
   JOIN   t2 ON t2.t1_id = path[1]
   LEFT   JOIN t1 USING (objid)
   )
SELECT DISTINCT ON (start_id)
       start_id, array_remove(path, NULL) AS path
FROM   cte
ORDER  BY start_id, array_length(path, 1) DESC;

Result:

start_id   path
1          {}
2          {}
3          {1,1}
4          {1,2}
5          {}
6          {}
7          {1,1,3,4,8,3}
8          {1,1,3,4}

array_remove() requires Postgres 9.3+.

Inverted the array to need one less columns. By putting the last element first, I can reference path[1] in the next step. Not sure if that's cheaper, would need a test ...

Shorter code, but probably slower. I suspect array handling is more expensive. Easier to adapt if we need to observe cycles.

SQL Fiddle.

Major points

• We are alternating between two tables.
  To make this recursive, one step needs to cover two hops (from t1 -> t2 and back t2 -> t1).

• The initial SELECT uses 2x LEFT JOIN to include all rows like in your example result.
  The recursive part JOIN to stop the loop when no match is found. The hop back uses LEFT JOIN again.

If I interpret your question correctly:

"Pick all rows where a has one of two given values, and b only exists in combination with one of them."

You can use any of the standard techniques laid out in the referenced answer.

• Select rows where value of second column is not present in first column

Just restrict your base table to the two given a. For instance

SELECT *
FROM   tbl t1
WHERE  a IN (1,2)
AND    NOT EXISTS (
   SELECT 1
   FROM   tbl t2
   WHERE  a IN (1,2)
   AND    t2.b =  t1.b
   AND    t2.a <> t1.a
   );

SQL Fiddle

This also works with duplicates on (a,b). Multiple identical rows would be returned.