Postgres 16

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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 TableA
(
    Id serial PRIMARY KEY,
    Name CHARACTER varying(30),
    tableB_Id INTEGER
);

insert into TableA VALUES (11, 'Name A', NULL), (22, 'Name B', NULL);

create table TableB
(
    Id serial PRIMARY KEY,
    Name CHARACTER varying(30),
    Descr CHARACTER(30)
);

INSERT INTO TableB VALUES 
  (123, 'Name A', 'Desc A'), 
  (456, 'Name X', 'Desc X'), 
  (789, 'Name B', 'Desc B');

CREATE TABLE

INSERT 0 2

CREATE TABLE

INSERT 0 3

UPDATE TableA 
SET tableB_Id = TableB.Id
FROM TableB
WHERE TableA.Name = TableB.Name;

UPDATE 2

SELECT * FROM TableA

id
name
tableb_id

11
Name A
123

22
Name B
789

SELECT 2