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?.

CREATE TABLE t_hour (duration interval hour);
CREATE TABLE t_sec  (duration interval second);
CREATE TABLE t_sec3 (duration interval second(3));
CREATE TABLE t_sec0 (duration interval second(0));

CREATE TABLE

CREATE TABLE

CREATE TABLE

CREATE TABLE

INSERT INTO t_hour VALUES ('13.556789') RETURNING *;

duration

13:00:00

INSERT 0 1

INSERT INTO t_sec  VALUES ('13.556789') RETURNING *;

duration

00:00:13.556789

INSERT 0 1

-- rounded!
INSERT INTO t_sec3  VALUES ('13.556789') RETURNING *;

duration

00:00:13.557

INSERT 0 1

-- rounded! (so unexpectedly results in 00:00:14)
INSERT INTO t_sec0  VALUES ('13.556789') RETURNING *;

duration

00:00:14

INSERT 0 1