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 sequence n_id;

CREATE TABLE IF NOT EXISTS currency
(
    id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
    name VARCHAR(30) UNIQUE NOT NULL,
    symbol VARCHAR(10) UNIQUE NOT NULL
);

CREATE TABLE IF NOT EXISTS stable (id INT default nextval('n_id') PRIMARY KEY) INHERITS (currency);    
CREATE TABLE IF NOT EXISTS crypto (id INT default nextval('n_id') PRIMARY KEY) INHERITS (currency);

CREATE SEQUENCE

CREATE TABLE

CREATE TABLE

CREATE TABLE

INSERT INTO stable (name, symbol) VALUES ('Euro', '€'), ('Dollar', '$'), ('Tether', 'USDT');    
INSERT INTO crypto (name, symbol) VALUES ('Bitcoin', 'BTC'), ('Ethereum', 'ETH'), ('Litcoin', 'LTC');

INSERT 0 3

INSERT 0 3

SELECT * FROM stable
UNION ALL
SELECT * FROM crypto

id
name
symbol

1
Euro
€

2
Dollar
$

3
Tether
USDT

4
Bitcoin
BTC

5
Ethereum
ETH

6
Litcoin
LTC

SELECT 6

id	name	symbol
1	Euro	€
2	Dollar	$
3	Tether	USDT
4	Bitcoin	BTC
5	Ethereum	ETH
6	Litcoin	LTC