How to return different format of records from a single PL/pgSQL function? - postgresql

Question

I am a frontend developer but I started to write backend stuff. I have spent quite some amount of time trying to figure out how to solve this. I really need some help.
Here are the simplified definitions and relations of two tables:
Relationship between tables
CREATE TABLE IF NOT EXISTS items (
item_id uuid NOT NULL DEFAULT gen_random_uuid() ,
parent_id uuid DEFAULT NULL ,
parent_table parent_tables NOT NULL
);
CREATE TABLE IF NOT EXISTS collections (
collection_id uuid NOT NULL DEFAULT gen_random_uuid() ,
parent_id uuid DEFAULT NULL
);
Our product is an online document collaboration tool, page can have nested pages.
I have a piece of PostgreSQL code for getting all of its ancestor records for given item_ids.
WITH RECURSIVE ancestors AS (
SELECT *
FROM items
WHERE item_id in ( ${itemIds} )
UNION
SELECT i.*
FROM items i
INNER JOIN ancestors a ON a.parent_id = i.item_id
)
SELECT * FROM ancestors
It works fine for nesting regular pages, But if I am going to support nesting collection pages, which means some items' parent_id might refer to "collection" table's collection_id, this code will not work anymore. According to my limited experience, I don't think pure SQL code can solve it. I think writing a PL/pgSQL function might be a solution, but I need to get all ancestor records to given itemIds, which means returning a mix of items and collections records.
So how to return different format of records from a single PL/pgSQL function? I did some research but haven't found any example.

Answer 1

You can make it work by returning a superset as row: comprised of item and collection. One of both will be NULL for each result row.
WITH RECURSIVE ancestors AS (
SELECT 0 AS lvl, i.parent_id, i.parent_table, i AS _item, NULL::collections AS _coll
FROM items i
WHERE item_id IN ( ${itemIds} )
UNION ALL -- !
SELECT lvl + 1, COALESCE(i.parent_id, c.parent_id), COALESCE(i.parent_table, 'i'), i, c
FROM ancestors a
LEFT JOIN items i ON a.parent_table = 'i' AND i.item_id = a.parent_id
LEFT JOIN collections c ON a.parent_table = 'c' AND c.collection_id = a.parent_id
WHERE a.parent_id IS NOT NULL
)
SELECT lvl, _item, _coll
FROM ancestors
-- ORDER BY ?
db<>fiddle here
UNION ALL, not UNION.
Assuming a collection's parent is always an item, while an item can go either way.
We need LEFT JOIN on both potential parent tables to stay in the race.
I added an optional lvl to keep track of the level of hierarchy.
About decomposing row types:
Combine postgres function with query
Record returned from function has columns concatenated