Postgres 12

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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

CREATE TABLE param (n INT NOT NULL, m INT NOT NULL);
INSERT INTO param (VALUES(3, 5)); SELECT * FROM param;
1 rows affected

n
m

3
5

SELECT
  UNNEST
  (
    ARRAY_CAT
    (
      ARRAY
      (
        SELECT GENERATE_SERIES(1, (SELECT n FROM param))::INT
      ), 
      ARRAY
      (
        SELECT GENERATE_SERIES((SELECT n FROM param), 1, -1)::INT
      )
    )
  ) FROM GENERATE_SERIES(1, (SELECT m FROM param));

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
  (
    ARRAY_CAT
    (
      ARRAY
      (
        SELECT GENERATE_SERIES(1, (SELECT n FROM param))::INT
      ), 
      ARRAY
      (
        SELECT GENERATE_SERIES((SELECT n FROM param), 1, -1)::INT
      )
    )
  ) FROM GENERATE_SERIES(1, (SELECT m FROM param));

QUERY PLAN

ProjectSet  (cost=107.84..177.84 rows=10000 width=4) (actual time=0.040..0.046 rows=30 loops=1)

  Output: unnest(array_cat($1, $3))

  Buffers: shared hit=3

  InitPlan 2 (returns $1)

    ->  ProjectSet  (cost=32.60..37.62 rows=1000 width=4) (actual time=0.007..0.008 rows=3 loops=1)

          Output: generate_series(1, $0)

          Buffers: shared hit=1

          InitPlan 1 (returns $0)

            ->  Seq Scan on public.param  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.004..0.005 rows=1 loops=1)

                  Output: param.n

                  Buffers: shared hit=1

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

  InitPlan 4 (returns $3)

    ->  ProjectSet  (cost=32.60..37.62 rows=1000 width=4) (actual time=0.010..0.011 rows=3 loops=1)

          Output: generate_series($2, 1, '-1'::integer)

          Buffers: shared hit=1

          InitPlan 3 (returns $2)

            ->  Seq Scan on public.param param_1  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.004..0.005 rows=1 loops=1)

                  Output: param_1.n

                  Buffers: shared hit=1

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

  InitPlan 5 (returns $4)

    ->  Seq Scan on public.param param_2  (cost=0.00..32.60 rows=2260 width=4) (actual time=0.007..0.007 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=0.017..0.018 rows=5 loops=1)

        Output: generate_series.generate_series

        Function Call: generate_series(1, $4)

        Buffers: shared hit=1

Planning Time: 0.110 ms

Execution Time: 0.092 ms