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 12.8 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.4.1 20200928 (Red Hat 8.4.1-1), 64-bit

--
-- Evan Carroll's solution
--
CREATE TABLE param (n INT, m INT);
INSERT INTO param (VALUES (3, 5)); 
SELECT * FROM param;
1 rows affected

n
m

3
5

SELECT x 
FROM 
(
  SELECT 1, ordinality AS o, x 
    FROM generate_series(1, (SELECT n FROM param)) 
      WITH ORDINALITY AS t(x)
  UNION ALL
  SELECT 2, ordinality AS o, x 
    FROM generate_series((SELECT n FROM param),1,-1) 
      WITH ORDINALITY AS t(x)
) AS t(o1, o2, x)
CROSS JOIN
(
  SELECT * FROM generate_series(1, (SELECT m FROM param))
) AS g(y)

x

1

2

3

3

2

1

1

2

3

3

2

1

1

2

3

3

2

1

1

2

3

3

2

1

1

2

3

3

2

1

EXPLAIN (ANALYZE, BUFFERS, VERBOSE)  -- ~ 0.125 ms
SELECT x 
FROM 
(
  SELECT 1, ordinality AS o, x 
    FROM generate_series(1, (SELECT n FROM param)) 
      WITH ORDINALITY AS t(x)
  UNION ALL
  SELECT 2, ordinality AS o, x 
    FROM generate_series((SELECT n FROM param),1,-1) 
      WITH ORDINALITY AS t(x)
) AS t(o1, o2, x)
CROSS JOIN
(
  SELECT * FROM generate_series(1, (SELECT m FROM param))
) AS g(y)

QUERY PLAN

Nested Loop  (cost=97.81..25142.81 rows=2000000 width=4) (actual time=0.044..0.073 rows=30 loops=1)

  Output: t.x

  Buffers: shared hit=3

  InitPlan 1 (returns $0)

    ->  Seq Scan on public.param  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.005..0.005 rows=1 loops=1)

          Output: param.n

          Buffers: shared hit=1

  InitPlan 2 (returns $1)

    ->  Seq Scan on public.param param_1  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.005..0.005 rows=1 loops=1)

          Output: param_1.n

          Buffers: shared hit=1

  InitPlan 3 (returns $2)

    ->  Seq Scan on public.param param_2  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.007..0.008 rows=1 loops=1)

          Output: param_2.m

          Buffers: shared hit=1

  ->  Function Scan on pg_catalog.generate_series  (cost=0.00..10.00 rows=1000 width=0) (actual time=0.023..0.023 rows=5 loops=1)

        Output: generate_series.generate_series

        Function Call: generate_series(1, $2)

        Buffers: shared hit=1

  ->  Materialize  (cost=0.00..40.00 rows=2000 width=4) (actual time=0.004..0.009 rows=6 loops=5)

        Output: t.x

        Buffers: shared hit=2

        ->  Append  (cost=0.00..30.00 rows=2000 width=4) (actual time=0.016..0.036 rows=6 loops=1)

              Buffers: shared hit=2

              ->  Function Scan on pg_catalog.generate_series t  (cost=0.00..10.00 rows=1000 width=4) (actual time=0.015..0.016 rows=3 loops=1)

                    Output: t.x

                    Function Call: generate_series(1, $0)

                    Buffers: shared hit=1

              ->  Function Scan on pg_catalog.generate_series t_1  (cost=0.00..10.00 rows=1000 width=4) (actual time=0.018..0.018 rows=3 loops=1)

                    Output: t_1.x

                    Function Call: generate_series($1, 1, '-1'::integer)

                    Buffers: shared hit=1

Planning Time: 0.135 ms

Execution Time: 0.128 ms