Postgres 12

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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 12.17 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-20), 64-bit

SELECT 1

create or replace function no_no_no()
  returns trigger
 language plpgsql
as $$
begin 
    raise exception 'Cannot update table %s.', tg_table_name;
    return null;
end; 
$$;

CREATE FUNCTION

create table NYAidCrimeMean( id integer generated always as identity 
                           , code text
                           ); 
 insert into NYAidCrimeMean (code) 
     select chr(ascii('A')+ n)
       from generate_series (0,25) gs(n) ; 

CREATE TABLE

INSERT 0 26

create trigger No_no_no_NYAidCrimeMeanA 
  before insert or update or delete or truncate on NYAidCrimeMean 
  for each statement 
  execute function no_no_no();

CREATE TRIGGER

update NYAidCrimeMean
   set code = code || ' 10-40';

ERROR:  Cannot update table nyaidcrimemeans.
CONTEXT:  PL/pgSQL function no_no_no() line 3 at RAISE