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?.

CREATE TABLE users (id int, school_id int, name varchar(32));

INSERT INTO users VALUES
  (42, 1, 'foo')
, (43, 2, 'bar');

CREATE TABLE



INSERT 0 2


CREATE OR REPLACE FUNCTION get_object_fields1(_school_id int
                                       , OUT user1_id   int
                                       , OUT user1_name varchar(32)
                                       , OUT user2_id   int
                                       , OUT user2_name varchar(32)) AS 
$func$
BEGIN
   SELECT INTO user1_id, user1_name
          u.id, u.name
   FROM   users u
   WHERE  u.school_id = _school_id;

   user2_id := user1_id + 1; -- some calculation

   SELECT INTO user2_name
          u.name       
   FROM   users u
   WHERE  u.id = user2_id;
END
$func$  LANGUAGE plpgsql;

SELECT * FROM get_object_fields1(1);


CREATE FUNCTION




user1_id
user1_name
user2_id
user2_name


42
foo
43
bar



SELECT 1


CREATE OR REPLACE FUNCTION get_object_fields2(_school_id int)
  RETURNS TABLE (user1_id   int
               , user1_name varchar(32)
               , user2_id   int
               , user2_name varchar(32)) AS 
$func$
BEGIN
   RETURN QUERY
   SELECT u1.id, u1.name, u2.id, u2.name
   FROM   users u1
   JOIN   users u2 ON u2.id = u1.id + 1
   WHERE  u1.school_id = _school_id
   LIMIT  1;  -- may be optional
END
$func$  LANGUAGE plpgsql;

SELECT * FROM get_object_fields2(1);


CREATE FUNCTION




user1_id
user1_name
user2_id
user2_name


42
foo
43
bar



SELECT 1