Partition pruning on parameters grouped into an array does not prune properly

Hi,

Working on improving partition pruning [1] I found a case that I may 
treat like a bug. I mean the case with expression on a partitioning 
column like:

id = ARRAY[$1,$2]

Basically, pruning on ARRAY[$1,$2] works in the case of single column 
(see correct-pruning-example.sql in attachment):

PREPARE test (int, int) AS
   SELECT * FROM array_prune WHERE id = ANY(ARRAY[$1,$2]);
EXPLAIN (COSTS OFF) EXECUTE test(1,2);

  Append
    Subplans Removed: 1
    ->  Seq Scan on array_prune_t0 array_prune_1
          Filter: (id = ANY (ARRAY[$1, $2]))

But if we partition on HASH(x,y) it is not working (see 
incorrect-pruning-example.sql):

PREPARE test2 (int,int) AS
  SELECT 1 FROM array_prune
  WHERE id1 = ANY(ARRAY[$1]) AND id2 = ANY(ARRAY[$2]);
EXPLAIN (COSTS OFF) EXECUTE test2(1,-1);

  Append
    ->  Seq Scan on array_prune_t0 array_prune_1
          Filter: ((id1 = ANY (ARRAY[$1])) AND (id2 = ANY (ARRAY[$2])))
    ->  Seq Scan on array_prune_t1 array_prune_2
          Filter: ((id1 = ANY (ARRAY[$1])) AND (id2 = ANY (ARRAY[$2])))

Although its analogue works nice:

PREPARE test3 (int,int) AS
  SELECT 1 FROM array_prune
  WHERE id1 = $1 AND id2 = $2;
EXPLAIN (COSTS OFF) EXECUTE test3(1,-1);

  Append
    Subplans Removed: 1
    ->  Seq Scan on array_prune_t0 array_prune_1
          Filter: ((id1 = $1) AND (id2 = $2))

So, before diving into the partitioning depths, someone may quickly say 
it is not a bug, but I am missing something. Some hidden semantics?


[1] Prune partitions by ScalarArrayOpExpr with an array parameter 
(partkey = ANY($1))
https://www.postgresql.org/message-id/b8cdd20f-b34b-42b9-8c7c-dae864b7b3b2@gmail.com

-- 
regards, Andrei Lepikhov