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?.

select version();

version

PostgreSQL 13.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.3.1 20191121 (Red Hat 8.3.1-5), 64-bit

CREATE DOMAIN ADDR TEXT NOT NULL
CHECK (LENGTH(VALUE) >= 5 AND LENGTH(VALUE) <= 40);
CREATE DOMAIN ffs TIMESTAMPTZ
CHECK (VALUE >= NOW());
CREATE TABLE fftest
(
  fff ffs default now()
);
insert into fftest (fff) values (default) ;
1 rows affected

select * from fftest ;

fff

2021-05-11 18:11:19.275618+01

insert into fftest (fff) values (now() - interval '1 second') ;
ERROR:  value for domain ffs violates check constraint "ffs_check"

select * from fftest ;

fff

2021-05-11 18:11:19.275618+01

insert into fftest (fff) values (default) ;
1 rows affected

select * from fftest ;

fff

2021-05-11 18:11:19.275618+01

2021-05-11 18:11:19.277164+01