Infinity recursive function exception in Postgresql - postgresql

Question

I have a table Answer and Many to Many table Link (Answer n-n Answer)
Link have 2 column : from_id and to_id reference to answer_id.
I want get all descendant of answer by answer_id ( from_id in Link ).
I have written function as below :
CREATE OR REPLACE FUNCTION getAllChild(_answer_id BIGINT)
RETURNS SETOF BIGINT AS $$
DECLARE r link;
BEGIN
FOR r IN
SELECT * FROM link
WHERE from_id = _answer_id
LOOP
RETURN NEXT r.to_id;
RETURN QUERY SELECT * FROM getAllChild(r.to_id);
END LOOP;
RETURN;
END;
$$ LANGUAGE plpgsql STRICT;
SELECT * FROM getAllChild(1);
The result is fine if to_id not duplicate with from_id that already got otherwise I will get recursive infinity.
My question is how I can make loop skip the existed to_id to call getAllChild() in RETURN QUERY

Answer 1

I'd suggest you do this with a recursive CTE, you could use the same approach in a function though.
You can use an array to keep a track of all the from_id's you've dealt with, and then in the next run through you ignore any records for from_id's already in the results. In the code below I'm using the path array to track all the from_id's already seen.
with recursive t as
(
select l.from_id,l.to_id, ARRAY[l.from_id] as path, 1 as depth
from link l where from_id = 2
union all
select l.from_id,l.to_id, array_append(t.path,l.from_id), t.depth+1
from link l
inner join t on l.from_id = t.to_id
where not (l.from_id = ANY (t.path)) -- ignore records already processed
)
select * from t;
Fiddle at: http://sqlfiddle.com/#!15/024e80/1
Updated: As a function
CREATE OR REPLACE FUNCTION getAllChild(_answer_id BIGINT)
RETURNS SETOF BIGINT AS $$
BEGIN
return query
with recursive t as
(
select l.from_id,l.to_id, ARRAY[l.from_id] as path, 1 as depth from link l where from_id = _answer_id
union all
select l.from_id,l.to_id, array_append(t.path,l.from_id), t.depth+1 from link l
inner join t on l.from_id = t.to_id
where not (l.from_id = ANY (t.path))
)
select to_id from t;
END;
$$ LANGUAGE plpgsql STRICT;
Arrays documentation: https://www.postgresql.org/docs/current/static/arrays.html
CTEs: https://www.postgresql.org/docs/current/static/queries-with.html