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?.
select version();


version


PostgreSQL 11.5 (Debian 11.5-3.pgdg90+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516, 64-bit


CREATE TABLE dictionary
(
  id INTEGER,
  first_level_key TEXT,
  key_name TEXT,
  key_value_en TEXT,
  key_value_fr TEXT
);
INSERT INTO dictionary 
VALUES 
(4, 'afasaffasa', 'ttttttt', 'zxccc', 'zzzzzzz'),
(4, 'afasawweww', 'rrrrrrr', 'zxccc', 'zzzzzzz'),
(4, 'afayyyyyyy', 'eeeeeee', 'zxccc', 'zzzzzzz'),
(4, 'xxxxxxxxxx', 'wwwwwww', 'zxccc', 'zzzzzzz'),
(4, 'mmmmmmmmmm', 'qqqqqqq', 'zxccc', 'zzzzzzz');
5 rows affected

SELECT JSONB_PRETTY(JSONB_AGG(tab)) AS i_json FROM
(
  SELECT
  ( 
    SELECT ROW_TO_JSON(x) 
    FROM 
    (
      SELECT c.first_level_key) AS x
    ) AS first_last,
    c.key_name, c.key_value_en
  FROM
  dictionary AS c
) AS tab;


i_json


[
    {
        "key_name": "ttttttt",
        "first_last": {
            "first_level_key": "afasaffasa"
        },
        "key_value_en": "zxccc"
    },
    {
        "key_name": "rrrrrrr",
        "first_last": {
            "first_level_key": "afasawweww"
        },
        "key_value_en": "zxccc"
    },
    {
        "key_name": "eeeeeee",
        "first_last": {
            "first_level_key": "afayyyyyyy"
        },
        "key_value_en": "zxccc"
    },
    {
        "key_name": "wwwwwww",
        "first_last": {
            "first_level_key": "xxxxxxxxxx"
        },
        "key_value_en": "zxccc"
    },
    {
        "key_name": "qqqqqqq",
        "first_last": {
            "first_level_key": "mmmmmmmmmm"
        },
        "key_value_en": "zxccc"
    }
]