Postgres 9.5

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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 9.5.18 on x86_64-pc-linux-gnu (Debian 9.5.18-1.pgdg80+1), compiled by gcc (Debian 4.9.2-10+deb8u2) 4.9.2, 64-bit

CREATE TABLE fred (mary int, paul varchar(20))
INSERT INTO fred VALUES (1, 'sdf'), (2, 'xxx'), (3, 'yyy')
3 rows affected

SELECT * FROM fred

mary
paul

1
sdf

2
xxx

3
yyy

SELECT * INTO TABLE bill FROM fred WHERE mary <= 2
2 rows affected

CREATE TABLE mark AS SELECT * FROM fred WHERE mary <= 2
2 rows affected

SELECT * FROM bill

mary
paul

1
sdf

2
xxx

SELECT * FROM mark

mary
paul

1
sdf

2
xxx