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 country(entity_id BIGINT, valid tsrange);

CREATE TABLE

INSERT INTO country 
VALUES(1, '[2016-06-22 19:10:25-07, 2019-01-01 00:00:00)')
,(1,'[2018-06-22 19:10:25-07, 2020-01-01 00:00:00)')

INSERT 0 2

CREATE OR REPLACE AGGREGATE range_merge(anyrange)
(
    sfunc = range_merge,
    stype = anyrange
);

CREATE AGGREGATE

CREATE OR REPLACE FUNCTION aggregate_validity(entity_name regclass, entry bigint) returns tsrange AS
$$
DECLARE
    result tsrange;
BEGIN
   EXECUTE format('select range_merge(valid) from %I where entity_id = %s', entity_name, entry) into result;
   return result;
END
$$ LANGUAGE plpgsql;

CREATE FUNCTION

select * from aggregate_validity('country', 1)

aggregate_validity

["2016-06-22 19:10:25","2020-01-01 00:00:00")

SELECT 1