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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 * FROM version();
| version |
|---|
| PostgreSQL 12.4 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.3.1 20191121 (Red Hat 8.3.1-5), 64-bit |
CREATE TABLE user_
(
user_id INTEGER NOT NULL PRIMARY KEY,
user_name VARCHAR (25) NOT NULL
);
INSERT INTO user_ (user_id, user_name)
SELECT 1, 'user_1'
UNION
SELECT 2, 'user_2';
2 rows affected
SELECT * FROM user_ ORDER BY user_id;
| user_id | user_name |
|---|---|
| 1 | user_1 |
| 2 | user_2 |
CREATE TABLE message
(
message_id INTEGER NOT NULL PRIMARY KEY,
sender_id INTEGER,
message_text VARCHAR (100) NOT NULL,
FOREIGN KEY (sender_id) REFERENCES user_ (user_id)
);
INSERT INTO message
SELECT 1, 1, 'm1'
UNION
SELECT 2, 1, 'm2'
UNION
SELECT 3, 2, 'm3'
UNION
SELECT 4, 2, 'm4';
4 rows affected
SELECT * FROM message ORDER BY message_id;
| message_id | sender_id | message_text |
|---|---|---|
| 1 | 1 | m1 |
| 2 | 1 | m2 |
| 3 | 2 | m3 |
| 4 | 2 | m4 |
INSERT INTO message VALUES (5, 56, 'mxxxx');
ERROR: insert or update on table "message" violates foreign key constraint "message_sender_id_fkey"
DETAIL: Key (sender_id)=(56) is not present in table "user_".