Re: WITH RECURSION output ordering with trees

Philippe Lang, 10.07.2009 11:10:
> Hi,
>
> I'm playing with the new "WITH RECURSIVE" feature of 8.4. I'm trying to
> figure out how to use it with trees.
>
> Here is the test code I use:
>
> I'd like to perform a real recursion, and show the tree structure in a
> more appopriate way, like this:
>
> Any idea how to do that? (without trying to sort on the lookup column,
> whose values can be random outside this test)

The manual has a nice hint on this adding up IDs to "generate" a path like column that can be used for sorting.

Try the following:

WITH RECURSIVE parse_tree (depth, id, lookup, parent_id, sort_path) AS
(
SELECT 0,
parent.id,
cast(parent.lookup as text),
parent.parent_id,
array[0] as sort_path
FROM recursion_sample parent
WHERE parent_id IS NULL
UNION ALL
SELECT
parent.depth + 1,
child.id,
rpad(' ', depth * 2) || child.lookup,
child.parent_id,
parent.sort_path || child.id
FROM parse_tree parent JOIN recursion_sample child on child.parent_id = parent.id
)
select id, lookup
from parse_tree
order by sort_path
;

This will output:

id | lookup
-----+--------
1 | a1
2 | b11
243 | c113
645 | c111
823 | c112
6 | b12
583 | c122
845 | c121
9 | b13
10 | c131
(10 rows)

Thomas