Postgres 10

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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?.

select version();

version

PostgreSQL 10.23 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-15), 64-bit

SELECT 1

CREATE TABLE my_table 
(
    the_debt_id varchar(6) NOT NULL, 
    the_debt_paid date NOT NULL, 
    the_debt_due date NOT NULL
);

INSERT INTO my_table
VALUES ('LMUS01', '2019-05-03', '2019-05-02'), 
       ('LMUS01', '2019-06-03', '2019-06-02'), 
       ('LMUS01', '2019-07-01', '2019-07-02'), 
       ('LMUS02', '2019-05-03', '2019-05-07'), 
       ('LMUS02', '2019-06-07', '2019-06-07')

CREATE TABLE

INSERT 0 5

SELECT * FROM my_table 
WHERE EXTRACT('month' from  the_debt_paid) = 6 and extract('year' from the_debt_paid)=2019

the_debt_id
the_debt_paid
the_debt_due

LMUS01
2019-06-03
2019-06-02

LMUS02
2019-06-07
2019-06-07

SELECT 2