Postgres 13

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

CREATE TABLE customer (customer_id INTEGER, name VARCHAR(6), passworddisabled VARCHAR(5));

INSERT INTO customer (customer_id, name, passworddisabled) VALUES
  ('1', 'Pedro', FALSE),
  ('2', 'Sandra', FALSE),
  ('3', 'Peter', FALSE),
  ('4', 'Norman', FALSE);

CREATE TABLE loan (loan_id INTEGER, loan_customerid INTEGER, loan_cashregister INTEGER);

INSERT INTO loan (loan_id, loan_customerid, loan_cashregister) VALUES
  ('1', '1', '1'),
  ('2', '1', '1'),
  ('3', '4', '2'),
  ('4', '1', '1');

CREATE TABLE

INSERT 0 4

CREATE TABLE

INSERT 0 4

UPDATE customer AS c
SET passworddisabled = EXISTS (
                         SELECT * 
                         FROM loan AS l 
                         WHERE c.customer_id = l.loan_customerid AND l.loan_cashregister = '1'
                       ); 

UPDATE 4

SELECT * FROM customer;

customer_id
name
passworddisabled

1
Pedro
true

2
Sandra
false

3
Peter
false

4
Norman
false

SELECT 4