Sorting rows by children? - postgresql

Question

I have this table:
CREATE TABLE items (
id SERIAL PRIMARY KEY,
data TEXT,
parent INT,
posted INT
);
Each item has a piece of data, a timestamp, and a parent. I'd like to select the top 10 root items (parent = 0), sorted by the timestamp of the most recent child.
If item #1 has a child #2 that has a child #3, #3 is considered a child of #1.
How can I do this?

Answer 1

EDIT:
The query has been rewritten to
first sort the child items
get the root parent id and the rank for each item
select the top 10 parents
select the details for the top 10 parents
Common Table expressions have been used to incrementally select the data following the above steps.
WITH recursive c AS
(
SELECT *
FROM seeds
UNION ALL
SELECT
T.id,
T.parent,
c.topParentID,
(c.child_level + 1),
c.child_rank
FROM items AS T
INNER JOIN c ON T.parent = c.id
WHERE T.id <> T.parent
)
, seeds AS
(
SELECT
id,
parent,
parent AS topParentID,
0 AS child_level,
rank() OVER (ORDER BY posted DESC) child_rank
FROM items
WHERE parent <> 0
ORDER BY posted DESC
)
, rank_level AS
(
SELECT DISTINCT
c2.id id,
c_ranks.min_child_rank child_rank,
c_roots.max_child_level root_level
FROM
(
SELECT
id,
MAX(child_level) max_child_level
FROM c
GROUP BY id
)
c_roots
INNER JOIN c c2 ON c_roots.id = c2.id
INNER JOIN
(
SELECT
id,
MIN(child_rank) min_child_rank
FROM c
GROUP BY id
)
c_ranks
ON c2.id = c_ranks.id
)
, top_10_parents AS
(
SELECT
c.topParentID id,
MIN(rl.child_rank) id_rank
FROM rank_level rl
INNER JOIN c ON rl.id = c.id AND c.child_level = rl.root_level
GROUP BY c.topParentID
ORDER BY MIN(rl.child_rank)
limit 10
)
SELECT
i.*
FROM
items i
INNER JOIN top_10_parents tp ON tp.id = i.id
ORDER BY tp.id_rank;
SQL Fiddle
Reference:
WITH Queries (Common Table Expressions) on PostgreSQL Manual