Postgres 11

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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?.

drop table if exists tasks;
create table tasks(id serial primary key, comments jsonb);
insert into tasks (comments) values
('[
	{
		"uid": "comment_1",
		"message": "level 1 - comment 1",
		"comments": [
			{
				"uid": "subcomment_1",
				"message": "level 2 - comment 1",
				"comments": []
			},
			{
				"uid": "subcomment_2",
				"message": "level 1 - comment 2",
				"comments": []
			}
		]
	},
	{
		"uid": "P7D1hbRq4",
		"message": "level 1 - comment 2",
		"comments": []
	}
]');
1 rows affected

update tasks
set comments = jsonb_set(comments, '{0, comments, 1, message}', '"comment edited"')
where id = 1;
1 rows affected

select jsonb_pretty(comments)
from tasks


jsonb_pretty


[
    {
        "uid": "comment_1",
        "message": "level 1 - comment 1",
        "comments": [
            {
                "uid": "subcomment_1",
                "message": "level 2 - comment 1",
                "comments": [
                ]
            },
            {
                "uid": "subcomment_2",
                "message": "comment edited",
                "comments": [
                ]
            }
        ]
    },
    {
        "uid": "P7D1hbRq4",
        "message": "level 1 - comment 2",
        "comments": [
        ]
    }
]