Postgres 10

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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 or replace FUNCTION p15d() 
  RETURNS table(date_ date ) AS $$ 
  declare 
  i date := current_date; 
  ii date := current_date + integer '15'; 
 BEGIN 
  loop 
      return query  select ( i + interval '1 day' )::date; 
      i = i + 1; 
      exit when i = ii; 
  end loop; 
 END; 
 $$ 
 LANGUAGE plpgsql; 
select p15d();

p15d

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CREATE or replace FUNCTION p15d() 
  RETURNS table(date_ date ) AS 
 $$ 
   select generate_series(current_date+1,current_date + 15,interval '1 day' )::date; 
 $$ 
 LANGUAGE SQL; 
select p15d();

p15d

2019-03-02

2019-03-03

2019-03-04

2019-03-05

2019-03-06

2019-03-07

2019-03-08

2019-03-09

2019-03-10

2019-03-11

2019-03-12

2019-03-13

2019-03-14

2019-03-15

2019-03-16