Re: Ltree - how to sort nodes on parent node

On 19 Apr 2010, at 20:26, cojack wrote:

>> Alban Hertroys wrote:
>>
>> It would help if you'd show us what result you expect from ordering the
>> above.
>>
>> Most people would order this by path I think. However that doesn't match
>> your sort column and I can't think of any method that would give results
>> in such an arbitrary order as you seem to be specifying - unless you set
>> it by hand like you do.

> Yes, you have right, for example I create new idea of stored data in table:
>
> here is a paste: http://pastebin.com/4pX5cM7j -- never expired link
>
> As you can see, I have noodes with numeric type, those nodes present a sort
> position by self. And If I type ORDER BY path; I will have data like I want
> to have: http://pastebin.com/R4z01LC5 -- never expired link
>
> Again, you can see now grouped data in his nodes, this is the outputed data
> I wanted. If you know better way to make this WITHOUT recursive queries,
> give me a hint.

Aha, looks like you want to sort each tree level by some user-specified order.

You should realise that ltree was contributed before Postgres supported (recursive) CTE's. If you're using ltree in combination with recursive CTE's you're doing twice the work that you need to do - ltree was created as a means to make recursive queries possible in the first place.

I think you have basically two ways to go about this:

1). The way you're doing this in your new examples should work, although I'd probably make the ordering numbers part of the category names and split those off when I read them. For example:
27 | 1|Top
28 | 1|Top.1|Science
29 | 1|Top.2|Hobby
30 | 1|Top.3|Colors
31 | 1|Top.1|Science.1|Physics
32 | 1|Top.1|Science.2|Chemistry
33 | 1|Top.1|Science.3|Biology
34 | 1|Top.1|Science.4|History
35 | 1|Top.2|Hobby.1|Fishing
36 | 1|Top.2|Hobby.2|Football
37 | 1|Top.3|Colors.1|Black
38 | 1|Top.3|Colors.2|Red
39 | 1|Top.3|Colors.3|Blue
40 | 1|Top.1|Science.5|Archeology
41 | 1|Top.2|Hobby.3|Swimming
42 | 1|Top.3|Colors.4|Gray
43 | 1|Top.3|Colors.5|Purple
44 | 1|Top.3|Colors.6|Brown
45 | 1|Top.2|Hobby.4|Climbing

2). Drop the ltree column and go with a truly recursive approach, something like this:

CREATE TABLE node (
category text NOT NULL PRIMARY KEY,
sort_order int NOT NULL,
parent text REFERENCES tree (category)
ON UPDATE CASCADE
ON DELETE CASCADE
);

WITH RECURSIVE tree AS (
SELECT *
FROM node
WHERE parent IS NULL

UNION ALL

SELECT node.*
FROM tree, node
WHERE node.parent = tree.category
ORDER BY sort_order
)
SELECT * FROM tree;

I haven't actually used recursive CTE's before so there may be some errors in the above, but you get the general idea.

Alban Hertroys

--
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