UNION pushdown issue when target list ordering is different in subquery

We currently look at the query level where a relation exists to understand the partition key index, however this might result in incorrect pushdown, for example:


Here `user_id` has an index of `1` in the subquery, (SELECT user_id, event_id ....).

However in the UNION we have its index as `2`, SELECT event_id, user_id FROM (SELECT .... )

So since Citus looks at the subquery, it thinks that partition key index is 1 in both sides of the UNION, and pushes down the query, which might give incorrect results if there are duplicates.