-- A scan whose plan asked for workers and got none (the pool of parallel workers is spent) runs in the leader alone: the rows of a correlated child
-- that the processes would have shared lived in the segment of the parallel context, which is gone, so the leader runs the child itself and its rows
-- are those of the normal plan.
CREATE EXTENSION pg_sift;
SET parallel_setup_cost = 0;
SET parallel_tuple_cost = 0;
SET min_parallel_table_scan_size = 0;
SET min_parallel_index_scan_size = 0;
SET seq_page_cost = 10000;
SET sift.agg_preagg = off;
SET sift.agg_par_min_rows = 0;
SET max_parallel_workers_per_gather = 3;
CREATE TABLE fl (id int, g int, h int, v float8) WITH (fillfactor = 10);
INSERT INTO fl SELECT i, i % 6000, i % 7, (i % 97) * 0.5 FROM generate_series(1, 20000) i;
CREATE INDEX fl_col ON fl USING sift_col (id, g, h, v);
NOTICE:  sift_col index "fl_col": 1 advice (track_changes)
DETAIL:  advice: track_changes = the table has no one-column primary key
HINT:  Reasons, costs and how to turn each off: SELECT * FROM sift.index_choices('fl_col');
CREATE TABLE l_d (h int, tag text);
INSERT INTO l_d SELECT i % 7, 't' || i FROM generate_series(1, 40) i;
VACUUM ANALYZE fl;
ANALYZE l_d;
CREATE TABLE n_q (q text);
INSERT INTO n_q VALUES ('SELECT f.h, c.tag, sum(f.v) AS s, count(*) AS n FROM fl f JOIN LATERAL (SELECT d.tag FROM l_d d WHERE d.h = f.h ORDER BY d.tag LIMIT 1) c ON true GROUP BY f.h, c.tag');
CREATE FUNCTION n_rows(push bool, avail int) RETURNS text LANGUAGE plpgsql AS $$
DECLARE r text;
BEGIN
	PERFORM set_config('sift.enable_agg', push::text, true);
	PERFORM set_config('max_parallel_workers', avail::text, true);
	EXECUTE format('SELECT string_agg(x::text, E''\n'' ORDER BY x::text) FROM (%s) x', (SELECT q FROM n_q)) INTO r;
	RETURN r;
END $$;
-- the plan asks for workers and is pushed down
CREATE FUNCTION n_plan(avail int) RETURNS text LANGUAGE plpgsql AS $$
DECLARE l text; pushed bool := false; planned bool := false;
BEGIN
	PERFORM set_config('max_parallel_workers', avail::text, true);
	FOR l IN EXECUTE 'EXPLAIN (COSTS OFF) ' || (SELECT q FROM n_q) LOOP
		planned := planned OR l ~ 'Workers Planned: [1-9]';
		pushed := pushed OR l ~ 'Custom Scan \(SiftAgg\)';
	END LOOP;
	RETURN format('pushed %s, workers planned %s', pushed, planned);
END $$;
SELECT n_plan(0) AS no_workers_available;
    no_workers_available     
-----------------------------
 pushed t, workers planned t
(1 row)

-- none can be launched: the rows are those of the normal plan, as they are when the workers start
SELECT n_rows(true, 0) = n_rows(false, 0) AS no_workers_equal;
 no_workers_equal 
------------------
 t
(1 row)

SELECT n_rows(true, 0) = n_rows(true, 8) AS with_workers_equal;
 with_workers_equal 
--------------------
 t
(1 row)

-- fewer workers than planned (the pool holds one of the three): the same rows
SELECT n_rows(true, 1) = n_rows(false, 1) AS one_worker_equal;
 one_worker_equal 
------------------
 t
(1 row)

DROP FUNCTION n_rows(bool, int), n_plan(int);
DROP TABLE n_q, fl, l_d;
DROP EXTENSION pg_sift;