get last node given the full path of all ancestor's node attributes using cte - sql

Question

Given the following PostgreSQL table:
items
integer id
integer parent_id
string name
unique key on [parent_id, name]
parent_id is null for all root nodes
Currently I build the sql query manually, doing a join for every path element. But is seems quite ugly to me and of course it limits the possible depth.
Example:
path: holiday,images,spain
SELECT i3.*
FROM items AS i1
, items AS i2
, items AS i3
WHERE i1.parent_id IS NULL AND i1.name = 'holiday'
AND i2.parent_id=i1.id AND i2.name = 'images'
AND i3.parent_id=i2.id AND i3.name = 'spain'
I wonder if there's a better way, probably using CTE?
You can see how my current code works and what the expected output is here:
http://sqlfiddle.com/#!1/4537c/2

Answer 1

update2 here's a function, it peforms well, because search goes only within the path, starting from parent:
create or replace function get_item(path text[])
returns items
as
$$
with recursive cte as (
select i.id, i.name, i.parent_id, 1 as level
from items as i
where i.parent_id is null and i.name = $1[1]
union all
select i.id, i.name, i.parent_id, c.level + 1
from items as i
inner join cte as c on c.id = i.parent_id
where i.name = $1[level + 1]
)
select c.id, c.parent_id, c.name
from cte as c
where c.level = array_length($1, 1)
$$
language sql;
sql fiddle demo
update I think you can do recursive traversal. I've written sql version of this, so it's a bit messy because of cte, but it's possible to write a function:
with recursive cte_path as (
select array['holiday', 'spain', '2013'] as arr
), cte as (
select i.id, i.name, i.parent_id, 1 as level
from items as i
cross join cte_path as p
where i.parent_id is null and name = p.arr[1]
union all
select i.id, i.name, i.parent_id, c.level + 1
from items as i
inner join cte as c on c.id = i.parent_id
cross join cte_path as p
where i.name = p.arr[level + 1]
)
select c.*
from cte as c
cross join cte_path as p
where level = array_length(p.arr, 1)
sql fiddle demo
or you can build path for all of the elements using recursive cte for that and accumuate your path into array or string:
with recursive cte as (
select i.id, i.name, i.parent_id, i.name::text as path
from items as i
where i.parent_id is null
union all
select i.id, i.name, i.parent_id, c.path || '->' || i.name::text as path
from items as i
inner join cte as c on c.id = i.parent_id
)
select *
from cte
where path = 'holiday->spain->2013';
or
with recursive cte as (
select i.id, i.name, i.parent_id, array[i.name::text] as path
from items as i
where i.parent_id is null
union all
select i.id, i.name, i.parent_id, c.path || array[i.name::text] as path
from items as i
inner join cte as c on c.id = i.parent_id
)
select *
from cte
where path = array['holiday', 'spain', '2013']
sql fiddle demo

Answer 2

This should perform very well, as it eliminates impossible paths immediately:
WITH RECURSIVE cte AS (
SELECT id, parent_id, name
,'{holiday,spain,2013}'::text[] AS path -- provide path as array here
,2 AS lvl -- next level
FROM items
WHERE parent_id IS NULL
AND name = 'holiday' -- being path[1]
UNION ALL
SELECT i.id, i.parent_id, i.name
,cte.path, cte.lvl + 1
FROM cte
JOIN items i ON i.parent_id = cte.id AND i.name = path[lvl]
)
SELECT id, parent_id, name
FROM cte
ORDER BY lvl DESC
LIMIT 1;
Assuming you provide a unique path (only 1 result).
->SQLfiddle demo

Answer 3

Too late to post my answer (very equivalent to Roman's and Erwin's) But an improvement on the table definition instead:
CREATE TABLE items
( id integer NOT NULL PRIMARY KEY
, parent_id integer REFERENCES items(id)
, name varchar
, UNIQUE (parent_id,name) -- I don't actually like this one
); -- ; UNIQUE on a NULLable column ...
INSERT INTO items (id, parent_id, name) values
(1, null, 'holiday')
, (2, 1, 'spain'), (3, 2, '2013')
, (4, 1, 'usa'), (5, 4, '2013')
;