PostgreSQL: Combining these two queries - sql

Question

My first query returns the first 10 comments, whose parent_id's are null
SELECT comment_id FROM comments WHERE thread_id = $1
AND parent_id is NULL
ORDER BY upvoted DESC FETCH FIRST 10 ROW ONLY
How would I use each comment id, to perform a recursive query?
My current method is by storing the returned comment_id's in an array
[1, 2, 3, 4, 10, 14, 15, 18, 19, 20]
and then using a for loop to perform a recursive query on each id
var query =
`With RECURSIVE cte AS
(
SELECT * FROM comments WHERE comment_id = $1
UNION
SELECT t.*
From comments t
JOIN cte rt ON rt.comment_id = t.parent_id
)
SELECT * FROM cte`;
for(int i = 0; i < array.size(); i++){
client.query(query, array[i])
. . .
}
So I am wondering how I could do this in a single query instead of storing the ids in an array and then recursive querying each individual id?

Answer 1

You can use your first statement as the non-recursive part of your recursive query. But due to the order by that you need, you need to put that query between parentheses:
With RECURSIVE cte AS
(
(
SELECT *
FROM comments
WHERE thread_id = $1
AND parent_id is NULL
ORDER BY upvoted DESC
FETCH FIRST 10 ROW ONLY
)
UNION
SELECT t.*
From comments t
JOIN cte rt ON rt.comment_id = t.parent_id
)
SELECT * FROM cte`
Alternatively you can do that in a separate CTE:
With RECURSIVE root_nodes AS
(
SELECT *
FROM comments
WHERE thread_id = $1
AND parent_id is NULL
ORDER BY upvoted DESC
FETCH FIRST 10 ROW ONLY
), cte as (
select *
from root_nodes
UNION
SELECT t.*
From comments t
JOIN cte rt ON rt.comment_id = t.parent_id
)
SELECT *
FROM cte;
Note the recursive keyword belongs to the WITH even when the first CTE is not the recursive one.