Thread: Parallel Append Query

Hi!

I'm seeing varying number of rows removed by filter on a parallel append query:

CREATE TABLE usuario (
  id bigint generated always as identity,
  nome text
);

INSERT INTO usuario (nome)
SELECT md5(seq)::text
    FROM generate_series(1,500000,1) tab(seq);

ANALYZE usuario;

EXPLAIN ANALYZE
(SELECT * FROM usuario WHERE nome = 'c4ca4238a0b923820dcc509a6f75849b')
UNION ALL
(SELECT * FROM usuario WHERE nome = 'c81e728d9d4c2f636f067f89cc14862c')

Gather  (cost=1000.00..15574.56 rows=2 width=41) (actual time=0.320..56.154 rows=2 loops=1)
  Workers Planned: 2
  Workers Launched: 2
  ->  Parallel Append  (cost=0.00..14574.36 rows=2 width=41) (actual time=13.449..47.899 rows=1 loops=3)
        ->  Parallel Seq Scan on usuario  (cost=0.00..7287.17 rows=1 width=41) (actual time=8.934..25.476 rows=0 loops=3)
              Filter: (nome = 'c81e728d9d4c2f636f067f89cc14862c'::text)
              Rows Removed by Filter: 166666
        ->  Parallel Seq Scan on usuario usuario_1  (cost=0.00..7287.17 rows=1 width=41) (actual time=13.470..33.630 rows=0 loops=2)
              Filter: (nome = 'c4ca4238a0b923820dcc509a6f75849b'::text)
              Rows Removed by Filter: 250000
Planning Time: 0.106 ms
Execution Time: 56.184 ms

The execution plan varies just by running the query a few times, and it goes back and forth between these two:

Gather  (cost=1000.00..15574.56 rows=2 width=41) (actual time=0.341..55.499 rows=2 loops=1)
  Workers Planned: 2
  Workers Launched: 2
  ->  Parallel Append  (cost=0.00..14574.36 rows=2 width=41) (actual time=15.904..49.821 rows=1 loops=3)
        ->  Parallel Seq Scan on usuario  (cost=0.00..7287.17 rows=1 width=41) (actual time=0.013..48.591 rows=1 loops=1)
              Filter: (nome = 'c81e728d9d4c2f636f067f89cc14862c'::text)
              Rows Removed by Filter: 499999
        ->  Parallel Seq Scan on usuario usuario_1  (cost=0.00..7287.17 rows=1 width=41) (actual time=15.985..33.621 rows=0 loops=3)
              Filter: (nome = 'c4ca4238a0b923820dcc509a6f75849b'::text)
              Rows Removed by Filter: 166666
Planning Time: 0.107 ms
Execution Time: 55.524 ms

But I can't understand why does it changes the number of loops from the queries