If this is a database for reporting, using a temporary table is probably fine and a union all wouldn’t concern me.

On Mysql, using a union all creates a temp table which can perform catastrophically under database load. I’ve seen a union all query with zero rows in the second half render a database server unresponsive when the database was under high load, causing a service disruption. We ended rewriting the union all query as two database fetches and have not seen a single problem in that area since.

I was shocked by this union all behavior, but it is apparently a well known thing on MySQL.

I can’t speak to Postgres behavior for this kind of query.