Re: BUG #15857: Parallel Hash Join makes join instead of exists

The following bug has been logged on the website:

Bug reference:      15857
Logged by:          Vladimir Kriukov
Email address:      krujkov@gmail.com
PostgreSQL version: 11.3
Operating system:   CentOS 7
Description:       

CREATE TABLE a(id int);
ALTER TABLE a ADD CONSTRAINT a_pkey PRIMARY KEY(id);
INSERT INTO a(id) SELECT generate_series(1, 1000000);
INSERT INTO a(id) SELECT generate_series(1000001, 10000000);
CREATE TABLE b(id int, base_id int);
ALTER TABLE b ADD CONSTRAINT b_pkey PRIMARY KEY(id);
INSERT INTO b (id) select generate_series(1, 1000000);
UPDATE b SET base_id = 1000000 - id;
CREATE TABLE c(id int, base_id int);
ALTER TABLE c ADD CONSTRAINT c_pkey PRIMARY KEY(id);
INSERT INTO c (id) SELECT generate_series(1, 1000000);
UPDATE c SET base_id = id / 10;

VACUUM ANALYZE;
SET random_page_cost = 1.1;
SET work_mem = '3276kB';
SET effective_cache_size = '90GB';

-- This gives an incorrect result of 999991, when 100000 is expected on
Postgres 11.3 and 12 beta 1.
SELECT COUNT (*)
  FROM a
  JOIN b
    ON a.id=b.base_id
 WHERE EXISTS (
       SELECT 1
         FROM c
        WHERE c.base_id = a.id
       );