Postgres 11

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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 to_date('2019-1-1', 'IYYY-IW-ID')

to_date

2018-12-31

SELECT to_date('2019-1-2', 'IYYY-IW-ID')

to_date

2019-01-01

SELECT to_date('2019-1-3', 'IYYY-IW-ID')

to_date

2019-01-02

SELECT to_date('2019-1-4', 'IYYY-IW-ID')

to_date

2019-01-03

SELECT to_date('2019-1-5', 'IYYY-IW-ID')

to_date

2019-01-04

SELECT to_date('2019-1-6', 'IYYY-IW-ID')

to_date

2019-01-05

SELECT to_date('2019-1-7', 'IYYY-IW-ID')

to_date

2019-01-06

SELECT to_date('2019-2-1', 'IYYY-IW-ID')

to_date

2019-01-07

SELECT to_date('2019-2-2', 'IYYY-IW-ID')

to_date

2019-01-08