Postgres: Nested records in a Recursive query in depth first manner - postgresql

Question

I am working on a simple comment system where a user can comment on other comments, thus creating a hierarchy. To get the comments in a hierarchical order I am using Common Table Expression in Postgres.
Below are the fields and the query used:
id
user_id
parent_comment_id
message
WITH RECURSIVE CommentCTE AS (
SELECT id, parent_comment_id, user_id
FROM comment
WHERE parent_comment_id is NULL
UNION ALL
SELECT child.id, child.parent_comment_id, child.user_id
FROM comment child
JOIN CommentCTE
ON child.parent_comment_id = CommentCTE.id
)
SELECT * FROM CommentCTE
The above query returns records in a breadth first manner:
id parent_comment_id user_id
10 null 30
9 null 30
11 9 30
14 10 31
15 10 31
12 11 30
13 12 31
But can it be modified to achieve something like below where records are returned together for that comment set, in a depth first manner? The point is to get the data in this way to make rendering on the Front-end smoother.
id parent_comment_id user_id
9 null 30
11 9 30
12 11 30
13 12 31
10 null 30
14 10 31
15 10 31

Answer 1

Generally I solve this problem by synthesising a "Path" column which can be sorted lexically, e.g. 0001:0003:0006:0009 is a child of 0001:0003:0006. Each child entry can be created by concatenating the path element to the parent's path. You don't have to return this column to the client, just use it for sorting.
id parent_comment_id user_id sort_key
9 null 30 0009
11 9 30 0009:0011
12 11 30 0009:0011:0012
13 12 31 0009:0011:0012:0013
10 null 30 0010
14 10 31 0010:0014
15 10 31 0010:0015
The path element doesn't have to be anything in particular provided it sorts lexically in the order you want children at that level to sort, and is unique at that level. Basing it on an auto-incrementing ID is fine.
Using a fixed length path element is not strictly speaking necessary but makes it easier to reason about.
WITH RECURSIVE CommentCTE AS (
SELECT id, parent_comment_id, user_id,
lpad(id::text, 4) sort_key
FROM comment
WHERE parent_comment_id is NULL
UNION ALL
SELECT child.id, child.parent_comment_id, child.user_id,
concat(CommentCTE.sort_key, ':', lpad(id::text, 4))
FROM comment child
JOIN CommentCTE
ON child.parent_comment_id = CommentCTE.id
)
SELECT * FROM CommentCTE order by sort_key