QUERY 1
EXPLAIN ANALYSE
SELECT
f.account_number
FROM
    financial_balances f

Seq Scan on financial_balances f  (cost=0.00..1579244.48 rows=5673748 width=10) (actual time=1.990..9970.375 rows=3452255 loops=1)
Planning time: 2.713 ms
Execution time: 10055.913 ms



QUERY 2
EXPLAIN ANALYSE
SELECT
 p.account_number
FROM
    debtor_personal P

Index Only Scan using debtor_personal_account_number_key on debtor_personal p  (cost=0.43..208808.44 rows=5636199 width=10) (actual time=0.191..817.714 rows=3452267 loops=1)
  Heap Fetches: 6812
Planning time: 1.086 ms
Execution time: 895.618 ms



QUERY 3
EXPLAIN ANALYSE
SELECT
f.account_number, p.account_number
FROM
    financial_balances f
INNER JOIN debtor_personal P ON f.account_number = f.account_number