Postgres 11

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select version();

version

PostgreSQL 11.13 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.4.1 20200928 (Red Hat 8.4.1-1), 64-bit

CREATE TABLE param(n INT, m INT);
INSERT INTO param (VALUES(3, 5));
SELECT * FROM param;
1 rows affected

n
m

3
5

SELECT UNNEST(arr)
FROM
(
  SELECT arr, GENERATE_SERIES(1, (SELECT m FROM param)) AS gs
  FROM
  (
    SELECT 
    ARRAY
    (
      SELECT x 
      FROM GENERATE_SERIES(1, (SELECT n FROM param)) x
      UNION ALL
      SELECT x
      FROM GENERATE_SERIES((SELECT n FROM param), 1, -1) x
    ) AS arr
  ) AS t
) AS s;

unnest

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)
SELECT UNNEST(arr)
FROM
(
  SELECT arr, GENERATE_SERIES(1, (SELECT m FROM param)) AS gs
  FROM
  (
    SELECT 
    ARRAY
    (
      SELECT x 
      FROM GENERATE_SERIES(1, (SELECT n FROM param)) x
      UNION ALL
      SELECT x
      FROM GENERATE_SERIES((SELECT n FROM param), 1, -1) x
    ) AS arr
  ) AS t
) AS s;

QUERY PLAN

ProjectSet  (cost=127.81..650.32 rows=100000 width=4) (actual time=0.040..0.046 rows=30 loops=1)

  Output: unnest(($2))

  Buffers: shared hit=3

  ->  ProjectSet  (cost=127.81..132.82 rows=1000 width=36) (actual time=0.039..0.041 rows=5 loops=1)

        Output: $2, generate_series(1, $3)

        Buffers: shared hit=3

        InitPlan 3 (returns $2)

          ->  Append  (cost=65.20..95.20 rows=2000 width=4) (actual time=0.018..0.031 rows=6 loops=1)

                Buffers: shared hit=2

                InitPlan 1 (returns $0)

                  ->  Seq Scan on public.param  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.006..0.007 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=0.004..0.004 rows=1 loops=1)

                        Output: param_1.n

                        Buffers: shared hit=1

                ->  Function Scan on pg_catalog.generate_series x  (cost=0.00..10.00 rows=1000 width=4) (actual time=0.017..0.018 rows=3 loops=1)

                      Output: x.x

                      Function Call: generate_series(1, $0)

                      Buffers: shared hit=1

                ->  Function Scan on pg_catalog.generate_series x_1  (cost=0.00..10.00 rows=1000 width=4) (actual time=0.011..0.011 rows=3 loops=1)

                      Output: x_1.x

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

                      Buffers: shared hit=1

        InitPlan 4 (returns $3)

          ->  Seq Scan on public.param param_2  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.004..0.004 rows=1 loops=1)

                Output: param_2.m

                Buffers: shared hit=1

        ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.000..0.001 rows=1 loops=1)

Planning Time: 0.152 ms

Execution Time: 0.111 ms