Empty results on large queries when using fetchall()

Hello there.

I've found that these two examples will not always return the same thing on large queries:

1. fetchall

cur.execute(large_query)
rows = cur.fetchall()
print(rows)

2. iterator

for r in cur.execute(large_query):
  print(r)

The fetchall approach ends up returning an empty list every time I use it when running large queries, but the iterator approach prints the result successfully

Does anyone know why this may be happening?

I don't know how I should supply a working example to debug this problem, as my queries all contain domain logic of my company. Nevertheless, I can say that I'm connecting around 8 Postgres databases with 100k rows per table and equal schemas, and I union the tables from all databases together with UNION views, like this:

CREATE OR REPLACE VIEW hive.default."table_unioned_view" AS 
SELECT 'database1' AS database, "other_column" FROM database1.public.table
UNION ALL
SELECT 'database2' AS database, "other_column" FROM database2.public.table
UNION ALL
SELECT 'database3' AS database, "other_column" FROM database3.public.table;

And the big query that I'm running uses these type of views

Start by comparing the cursor.execute/fetchall path with cursor iteration using a minimal large-query example, since the report cannot provide a reproducible query. Check whether the behavior depends on the UNION views across the eight PostgreSQL databases; done means identifying and reproducing the cause of the differing results.