Postgres 9.6

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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 the_table (id int, array_json jsonb);

CREATE TABLE

insert into the_table
values
(123, '[{"end_time": 22, "start_time": 21}]'),
(124, '[{"end_time": 22, "start_time": 9}, {"end_time": 20, "start_time": 7}]'),
(123, '[{"end_time": 22, "start_time": 0}]'),
(125, '[{"end_time": 22, "start_time": 1}, {"end_time": 20, "start_time": 6}]');

INSERT 0 4

select * from the_table;

id
array_json

123
[{"end_time": 22, "start_time": 21}]

124
[{"end_time": 22, "start_time": 9}, {"end_time": 20, "start_time": 7}]

123
[{"end_time": 22, "start_time": 0}]

125
[{"end_time": 22, "start_time": 1}, {"end_time": 20, "start_time": 6}]

SELECT 4

id	array_json
123	[{"end_time": 22, "start_time": 21}]
124	[{"end_time": 22, "start_time": 9}, {"end_time": 20, "start_time": 7}]
123	[{"end_time": 22, "start_time": 0}]
125	[{"end_time": 22, "start_time": 1}, {"end_time": 20, "start_time": 6}]