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

n
m

3
5

WITH cte AS
(
  SELECT
    ARRAY(SELECT GENERATE_SERIES(1, 3)) AS arr
)
SELECT ARRAY_CAT(c.arr, SORT(c.arr, 'DESC'))
FROM cte c;

array_cat

{1,2,3,3,2,1}

WITH cte AS
(
  SELECT
    ARRAY(SELECT GENERATE_SERIES(1, (SELECT n FROM param))) AS arr
)
SELECT UNNEST 
(
  (
    SELECT ARRAY_CAT(c.arr, SORT(c.arr, 'DESC'))
    FROM cte c
  )
) 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)
WITH cte AS
(
  SELECT
    ARRAY(SELECT GENERATE_SERIES(1, (SELECT n FROM param))) AS arr
)
SELECT UNNEST 
(
  (
    SELECT ARRAY_CAT(c.arr, SORT(c.arr, 'DESC'))
    FROM cte c
  )
) FROM GENERATE_SERIES(1, (SELECT m FROM param));

QUERY PLAN

ProjectSet  (cost=107.85..175.35 rows=10000 width=4) (actual time=0.042..0.048 rows=30 loops=1)

  Output: unnest($4)

  Buffers: shared hit=3

  InitPlan 5 (returns $4)

    ->  Result  (cost=75.23..75.25 rows=1 width=32) (actual time=0.022..0.023 rows=1 loops=1)

          Output: array_cat($1, sort($3, 'DESC'::text))

          Buffers: shared hit=2

          InitPlan 2 (returns $1)

            ->  ProjectSet  (cost=32.60..37.62 rows=1000 width=4) (actual time=0.010..0.011 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=0.006..0.007 rows=3 loops=1)

                  Output: generate_series(1, $2)

                  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..0.004 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 6 (returns $5)

    ->  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=0.018..0.018 rows=5 loops=1)

        Output: generate_series.generate_series

        Function Call: generate_series(1, $5)

        Buffers: shared hit=1

Planning Time: 0.154 ms

Execution Time: 0.112 ms