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_sampleparent  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
childon child.parent_id = parent.id
 
)
select id, lookup
from parse_tree
order by sort_path
;

This will output:
id  | lookup
-----+--------  1 | a1  2 | b11243 |   c113645 |   c111823 |   c112  6 | b12583 |   c122845 |   c121  9 | b13 10 |
c131
(10 rows)

Thomas