pg13dev: explain partial, parallel hashagg, and memory use

[Justin Pryzby]

I'm testing with a customer's data on pg13dev and got output for which Peak
Memory doesn't look right/useful.  I reproduced it on 565f16902.

CREATE TABLE p(i int) PARTITION BY RANGE(i);
CREATE TABLE p1 PARTITION OF p FOR VALUES FROM (0)TO(1000);
CREATE TABLE p2 PARTITION OF p FOR VALUES FROM (1000)TO(2000);
CREATE TABLE p3 PARTITION OF p FOR VALUES FROM (2000)TO(3000);
INSERT INTO p SELECT i%3000 FROM generate_series(1,999999)i;
VACUUM ANALYZE p;

postgres=# explain(analyze,settings) SELECT i, COUNT(1) FROM p GROUP BY 1;
                                                              QUERY PLAN        
                                                       
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=7469.00..14214.45 rows=2502 width=12) (actual 
time=489.409..514.209 rows=3000 loops=1)
   Workers Planned: 2
   Workers Launched: 2
   ->  Parallel Append  (cost=6469.00..12964.25 rows=1251 width=12) (actual 
time=476.291..477.179 rows=1000 loops=3)
         ->  HashAggregate  (cost=6487.99..6497.99 rows=1000 width=12) (actual 
time=474.454..475.203 rows=1000 loops=1)
               Group Key: p.i
               Batches: 1  Memory Usage: 0kB
               Worker 0:  Batches: 1  Memory Usage: 193kB
               Worker 1:  Batches: 1  Memory Usage: 0kB
               ->  Seq Scan on p1 p  (cost=0.00..4817.99 rows=333999 width=4) 
(actual time=0.084..100.677 rows=333999 loops=1)
         ->  HashAggregate  (cost=6469.00..6479.00 rows=1000 width=12) (actual 
time=468.517..469.272 rows=1000 loops=1)
               Group Key: p_1.i
               Batches: 1  Memory Usage: 0kB
               Worker 0:  Batches: 1  Memory Usage: 0kB
               Worker 1:  Batches: 1  Memory Usage: 193kB
               ->  Seq Scan on p2 p_1  (cost=0.00..4804.00 rows=333000 width=4) 
(actual time=0.082..102.154 rows=333000 loops=1)
         ->  HashAggregate  (cost=6469.00..6479.00 rows=1000 width=12) (actual 
time=485.887..486.509 rows=1000 loops=1)
               Group Key: p_2.i
               Batches: 1  Memory Usage: 193kB
               Worker 0:  Batches: 1  Memory Usage: 0kB
               Worker 1:  Batches: 1  Memory Usage: 0kB
               ->  Seq Scan on p3 p_2  (cost=0.00..4804.00 rows=333000 width=4) 
(actual time=0.043..104.631 rows=333000 loops=1)
 Settings: effective_io_concurrency = '0', enable_partitionwise_aggregate = 
'on', enable_partitionwise_join = 'on', work_mem = '127MB'

-- 
Justin