Bug report

repro.sql

-- Test correctness of parallel query execution after removal
-- of Append path due to single non-trivial child.

-- It is (were) broken since commit 8edd0e79460b414b1d971895312e549e95e12e4f
-- "Suppress Append and MergeAppend plan nodes that have a single child."
-- ie PostgreSQL v12.

-- Plans below for broken version.

DROP TABLE IF EXISTS gather_append_1, gather_append_2;

CREATE TABLE gather_append_1 (
    fk int,
    f bool
);

INSERT INTO gather_append_1 (fk, f) SELECT i, i%50=0 from generate_series(1, 2000) as i;

CREATE INDEX gather_append_1_ix on gather_append_1 (f);

CREATE TABLE gather_append_2 (
    fk int,
    val serial
);

INSERT INTO gather_append_2 (fk) SELECT fk from gather_append_1, generate_series(1, 5) as i;

ANALYZE gather_append_1, gather_append_2;

SET max_parallel_workers_per_gather = 0;

-- Find correct rows count
SELECT count(1)
FROM (
  SELECT fk FROM gather_append_1 WHERE f
  UNION ALL
  SELECT fk FROM gather_append_1 WHERE false
) as t
LEFT OUTER JOIN gather_append_2
USING (fk);
-- count
---------
--   200
--(1 row)

SET parallel_setup_cost = 0;
SET parallel_tuple_cost = 0.1;
SET min_parallel_table_scan_size = 0;
SET max_parallel_workers_per_gather = 2;

SELECT count(1)
FROM (
  SELECT fk FROM gather_append_1 WHERE f
  UNION ALL
  SELECT fk FROM gather_append_1 WHERE false
) as t
LEFT OUTER JOIN gather_append_2
USING (fk);
-- count
---------
--   400
--(1 row)

EXPLAIN (ANALYZE true, COSTS false, TIMING false, SUMMARY false, VERBOSE false, BUFFERS false)
SELECT count(1)
FROM (
  SELECT fk FROM gather_append_1 WHERE f
  UNION ALL
  SELECT fk FROM gather_append_1 WHERE false
) as t
LEFT OUTER JOIN gather_append_2
USING (fk);

-- Finalize Aggregate (actual rows=1 loops=1)
--   ->  Gather (actual rows=2 loops=1)
--         Workers Planned: 1
--         Workers Launched: 1
--         ->  Partial Aggregate (actual rows=1 loops=2)
--               ->  Parallel Hash Left Join (actual rows=200 loops=2)
--                     Hash Cond: (gather_append_1.fk = gather_append_2.fk)
--                     ->  Index Scan using gather_append_1_ix on gather_append_1 (actual rows=40 loops=2)
--                           Index Cond: (f = true)
--                     ->  Parallel Hash (actual rows=5000 loops=2)
--                           Buckets: 16384  Batches: 1  Memory Usage: 544kB
--                           ->  Parallel Seq Scan on gather_append_2 (actual rows=5000 loops=2)


-- Result rows in root node should be equal to non-parallel count
EXPLAIN (ANALYZE true, COSTS false, TIMING false, SUMMARY false, VERBOSE false, BUFFERS false)
SELECT val
FROM (
  SELECT fk FROM gather_append_1 WHERE f
  UNION ALL
  SELECT fk FROM gather_append_1 WHERE false
) as t
LEFT OUTER JOIN gather_append_2
USING (fk)
ORDER BY val;

-- Gather Merge (actual rows=400 loops=1)
--   Workers Planned: 1
--   Workers Launched: 1
--   ->  Sort (actual rows=200 loops=2)
--         Sort Key: gather_append_2.val
--         Sort Method: quicksort  Memory: 25kB
--         Worker 0:  Sort Method: quicksort  Memory: 25kB
--         ->  Parallel Hash Left Join (actual rows=200 loops=2)
--               Hash Cond: (gather_append_1.fk = gather_append_2.fk)
--               ->  Index Scan using gather_append_1_ix on gather_append_1 (actual rows=40 loops=2)
--                     Index Cond: (f = true)
--               ->  Parallel Hash (actual rows=5000 loops=2)
--                     Buckets: 16384  Batches: 1  Memory Usage: 544kB
--                     ->  Parallel Seq Scan on gather_append_2 (actual rows=5000 loops=2)

-- Result rows in root node should be equal to non-parallel count
EXPLAIN (ANALYZE true, COSTS false, TIMING false, SUMMARY false, VERBOSE false, BUFFERS false)
SELECT val
FROM (
  SELECT fk FROM gather_append_1 WHERE f
  UNION ALL
  SELECT fk FROM gather_append_1 WHERE false
) as t
LEFT OUTER JOIN gather_append_2
USING (fk)
ORDER BY val;

-- Gather Merge (actual rows=400 loops=1)
--   Workers Planned: 1
--   Workers Launched: 1
--   ->  Sort (actual rows=200 loops=2)
--         Sort Key: gather_append_2.val
--         Sort Method: quicksort  Memory: 25kB
--         Worker 0:  Sort Method: quicksort  Memory: 25kB
--         ->  Parallel Hash Left Join (actual rows=200 loops=2)
--               Hash Cond: (gather_append_1.fk = gather_append_2.fk)
--               ->  Index Scan using gather_append_1_ix on gather_append_1 (actual rows=40 loops=2)
--                     Index Cond: (f = true)
--               ->  Parallel Hash (actual rows=5000 loops=2)
--                     Buckets: 16384  Batches: 1  Memory Usage: 544kB
--                     ->  Parallel Seq Scan on gather_append_2 (actual rows=5000 loops=2)