behavior of GROUP BY with VOLATILE expressions

Hackers,

I wanted to surface a discussion in [1] regarding the expected behavior of GROUP BY with VOLATILE expressions. There seems to be a discrepancy between how volatile functions (RANDOM(), also confirmed with TIMEOFDAY()) and subqueries are evaluated in groups. In the examples below, volatile functions do not always appear to be evaluated per-call (evidenced by looking at EXPLAIN or results) whereas scalar subqueries always appear to be independently evaluated.

[version: PostgreSQL 16.3]

-- ### grouping volatile functions

-- single evaluation of RANDOM()
select random(), random(), random() group by 1;
       random       |       random       |       random      
--------------------+--------------------+--------------------
 0.5156775158087117 | 0.5156775158087117 | 0.5156775158087117
(1 row)

-- two evaluations of RANDOM()

select random(), random(), random() group by 1, 2;
       random        |       random        |       random        
---------------------+---------------------+---------------------
 0.36612763448670793 | 0.23423805164449374 | 0.36612763448670793
(1 row)

-- three evaluations of RANDOM()
select random(), random(), random() group by 1, 2, 3;
       random       |       random       |       random      
--------------------+--------------------+--------------------
 0.2292929455776751 | 0.6613628224046473 | 0.5367692073422399
(1 row)


-- single evaluation of RANDOM()
select random(), random(), random() group by random(), random();
       random       |       random       |       random      
--------------------+--------------------+--------------------
 0.2860459945718521 | 0.2860459945718521 | 0.2860459945718521
(1 row)

-- single evaluation of RANDOM()

select random(), random(), random() group by random(), random(), random();
       random       |       random       |       random      
--------------------+--------------------+--------------------
 0.3249129391658361 | 0.3249129391658361 | 0.3249129391658361
(1 row)

-- ### grouping scalar subqueries

-- each subquery evaluated

select (select random()), (select random()), (select random()) group by 1;
       random        |       random       |       random      
---------------------+--------------------+--------------------
 0.30149979064538757 | 0.7911979526441186 | 0.5251471322291046
(1 row)

-- each subquery evaluated

select (select random()), (select random()), (select random()) group by (select random());
       random       |       random       |        random        
--------------------+--------------------+----------------------
 0.3411533489925591 | 0.4359004781684166 | 0.018305770511828356
(1 row)

-- ### sample EXPLAINs

-- two evaluations of RANDOM()

explain (verbose, costs off) select random(), random(), random() group by 1, 2;
                  QUERY PLAN                  
----------------------------------------------
 HashAggregate
   Output: (random()), (random()), (random())
   Group Key: random(), random()
   ->  Result
         Output: random(), random()
(5 rows)

-- singe evaluation of RANDOM()

explain (verbose, costs off) select random(), random(), random() group by random(), random();
                  QUERY PLAN                  
----------------------------------------------
 HashAggregate
   Output: (random()), (random()), (random())
   Group Key: random()
   ->  Result
         Output: random()
(5 rows)

-Paul-

[1] https://www.postgresql.org/message-id/CAMbWs4_xhRAfy-i%3D%3DnFMZGukw4M%3DOnkfwpfEfiGmAx6a3SYBKw%40mail.gmail.com