--sql

CREATE TABLE geoms (geom geometry);

-- Create 20x20 square polygons
WITH coords AS (SELECT * FROM generate_series(0, 19) AS x, generate_series(0, 19) AS y)
INSERT INTO geoms
SELECT ST_Square(1.0, x, y, ST_MakePoint(0, 0))
FROM coords;


SET force_parallel_mode=ON;
SET parallel_setup_cost = 0;
SET parallel_tuple_cost = 0;
SET min_parallel_table_scan_size = 0;
SET max_parallel_workers_per_gather = 6;

explain analyze WITH u AS (SELECT ST_Union(geom) AS g FROM geoms)
SELECT ST_Area(g), ST_XMin((g)), ST_YMin(g), ST_XMax(g), ST_YMax(g) FROM u;


-- version info

POSTGIS="3.2.0 c3e3cc0" [EXTENSION] PGSQL="130" GEOS="3.9.0-CAPI-1.16.2" PROJ="7.2.1" LIBXML="2.9.10" LIBJSON="0.15" LIBPROTOBUF="1.3.3" WAGYU="0.5.0 (Internal)" TOPOLOGY

POSTGIS="3.3.4 3.3.4" [EXTENSION] PGSQL="130" GEOS="3.9.0-CAPI-1.16.2" PROJ="7.2.1" LIBXML="2.9.10" LIBJSON="0.15" LIBPROTOBUF="1.3.3" WAGYU="0.5.0 (Internal)" TOPOLOGY


-- explain output

postgis 3.2

QUERY PLAN                                                                                                                    |
------------------------------------------------------------------------------------------------------------------------------+
Subquery Scan ON u  (cost=84.67..85.32 rows=1 width=40) (actual TIME=18.433..18.465 rows=1 loops=1)                           |
  ->  Aggregate  (cost=84.67..84.68 rows=1 width=32) (actual TIME=18.429..18.460 rows=1 loops=1)                              |
        ->  Gather  (cost=0.00..9.67 rows=400 width=120) (actual TIME=0.150..8.434 rows=400 loops=1)                          |
              Workers Planned: 2                                                                                              |
              Workers Launched: 2                                                                                             |
              ->  Parallel Seq Scan ON geoms  (cost=0.00..9.67 rows=167 width=120) (actual TIME=0.002..0.019 rows=133 loops=3)|
Planning TIME: 0.036 ms                                                                                                       |
Execution TIME: 18.524 ms                                                                                                     |


postgis3.3

QUERY PLAN                                                                                                                          |
------------------------------------------------------------------------------------------------------------------------------------+
Subquery Scan ON u  (cost=35.11..35.76 rows=1 width=40) (actual TIME=18.839..18.870 rows=1 loops=1)                                 |
  ->  Finalize Aggregate  (cost=35.11..35.12 rows=1 width=32) (actual TIME=18.835..18.866 rows=1 loops=1)                           |
        ->  Gather  (cost=10.09..10.10 rows=2 width=32) (actual TIME=0.294..8.671 rows=3 loops=1)                                   |
              Workers Planned: 2                                                                                                    |
              Workers Launched: 2                                                                                                   |
              ->  Partial Aggregate  (cost=10.09..10.10 rows=1 width=32) (actual TIME=0.048..0.049 rows=1 loops=3)                  |
                    ->  Parallel Seq Scan ON geoms  (cost=0.00..9.67 rows=167 width=120) (actual TIME=0.007..0.017 rows=133 loops=3)|
Planning TIME: 0.041 ms                                                                                                             |
Execution TIME: 18.927 ms                                                                                                           |