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?.

    CREATE TABLE sample
    (
      x INTEGER,    -- always have to specify a non-null value for x and y!
      y VARCHAR(10),
      z INTEGER NOT NULL DEFAULT 3,
      w TIMESTAMP,
      v TIMESTAMP NOT NULL DEFAULT NOW()
    );
INSERT INTO sample (x) VALUES (34);
1 rows affected

INSERT INTO sample (y) VALUES (34), ('a') ;
2 rows affected

INSERT INTO sample (z) VALUES (34);
1 rows affected

INSERT INTO sample SELECT;
1 rows affected

SELECT * FROM sample;


x
y
z
w
v


34
null
3
null
2018-05-21 13:02:23.603088

null
34
3
null
2018-05-21 13:02:23.606638

null
a
3
null
2018-05-21 13:02:23.606638

null
null
34
null
2018-05-21 13:02:23.609989

null
null
3
null
2018-05-21 13:02:23.613076

x	y	z	w	v
34	null	3	null	2018-05-21 13:02:23.603088
null	34	3	null	2018-05-21 13:02:23.606638
null	a	3	null	2018-05-21 13:02:23.606638
null	null	34	null	2018-05-21 13:02:23.609989
null	null	3	null	2018-05-21 13:02:23.613076