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?.

select version();

version

PostgreSQL 16.0 (Debian 16.0-1.pgdg120+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit

SELECT 1

create table foo( id  serial
                , a   text
                , x   text    
                ); 
               
insert into foo( a, x) 
   values ('Change me at will', 'Cannot change me')
        , ('Change me too', null);

select *
  from foo; 

CREATE TABLE

INSERT 0 2

id
a
x

1
Change me at will
Cannot change me

2
Change me too
null

SELECT 2

create or replace function make_x_immutable() 
   returns trigger 
   language plpgsql
as $$
begin 
   new.x = old.x;
   return new;
end; 
$$;

create trigger  foo_bur
    before update on foo
    for each row 
    execute function make_x_immutable() ; 

CREATE FUNCTION

CREATE TRIGGER

update foo 
   set a = 'OK ! I will'
     , x = 'Oh Yea. Just watch';
    
select *
  from foo;

UPDATE 2

id
a
x

1
OK ! I will
Cannot change me

2
OK ! I will
null

SELECT 2