Recursive CTE concatenate fields with parents from arbitrary point

Question

How can I concatenate a list of parent items in a RECURSIVE CTE with PostgreSQL (version 10.2)?

For example, I have:

CREATE TABLE test (
    id SERIA         


        

Answer 1

In the top-down method the initial query should select only roots (items without parents), so the query returns each row only once:

with recursive top_down as (
    select id, parent, text
    from test
    where parent is null
union all
    select t.id, t.parent, concat_ws('/', r.text, t.text)
    from test t
    join top_down r on t.parent = r.id
)
select id, text
from top_down
where id = 4    -- input

If your goal is to find a specific item, the bottom-up approach is more efficient:

with recursive bottom_up as (
    select id, parent, text
    from test
    where id = 4    -- input
union all
    select r.id, t.parent, concat_ws('/', t.text, r.text)
    from test t
    join bottom_up r on r.parent = t.id
)
select id, text
from bottom_up
where parent is null

Remove final where conditions in both queries to see the difference.

Test it in rextester.