Postgres 13

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select version();

version

PostgreSQL 13.4 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
  gn.i
from
  ( with g(i) as
    ( select * from generate_series(1, (SELECT n FROM param)) )
    (select i from g)
    union all
    (select i from g order by i desc)
  ) as gn,
  generate_series(1, (SELECT m FROM param)) as gm (j) ;

i

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
  gn.i
from
  ( with g(i) as
    ( select * from generate_series(1, (SELECT n FROM param)) )
    (select i from g)
    union all
    (select i from g order by i desc)
  ) as gn,
  generate_series(1, (SELECT m FROM param)) as gm (j) ;

QUERY PLAN

Nested Loop  (cost=75.20..25222.53 rows=2000000 width=4) (actual time=0.033..0.058 rows=30 loops=1)

  Output: g.i

  Buffers: shared hit=2

  InitPlan 1 (returns $0)

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

          Output: param.m

          Buffers: shared hit=1

  ->  Function Scan on pg_catalog.generate_series gm  (cost=0.00..10.00 rows=1000 width=0) (actual time=0.018..0.018 rows=5 loops=1)

        Output: gm.j

        Function Call: generate_series(1, $0)

        Buffers: shared hit=1

  ->  Materialize  (cost=42.60..184.93 rows=2000 width=4) (actual time=0.003..0.007 rows=6 loops=5)

        Output: g.i

        Buffers: shared hit=1

        ->  Append  (cost=42.60..154.93 rows=2000 width=4) (actual time=0.014..0.030 rows=6 loops=1)

              Buffers: shared hit=1

              CTE g

                ->  Function Scan on pg_catalog.generate_series  (cost=32.60..42.60 rows=1000 width=4) (actual time=0.012..0.013 rows=3 loops=1)

                      Output: generate_series.generate_series

                      Function Call: generate_series(1, $1)

                      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

              ->  CTE Scan on g  (cost=0.00..20.00 rows=1000 width=4) (actual time=0.013..0.014 rows=3 loops=1)

                    Output: g.i

                    Buffers: shared hit=1

              ->  Sort  (cost=69.83..72.33 rows=1000 width=4) (actual time=0.014..0.014 rows=3 loops=1)

                    Output: g_1.i

                    Sort Key: g_1.i DESC

                    Sort Method: quicksort  Memory: 25kB

                    ->  CTE Scan on g g_1  (cost=0.00..20.00 rows=1000 width=4) (actual time=0.001..0.004 rows=3 loops=1)

                          Output: g_1.i

Planning Time: 0.144 ms

Execution Time: 0.089 ms