Runtime partition pruning with hash partitioning

Hello,

Runtime partition pruning don't work without index on a hash partitioned 
column.
Consider this test case on version 12:

create table data (
   key_id    integer not null,
   value     real not null
) partition by hash(key_id);

create table data_0 partition of data for values with (modulus 3, 
remainder 0);
create table data_1 partition of data for values with (modulus 3, 
remainder 1);
create table data_2 partition of data for values with (modulus 3, 
remainder 2);

insert into data (key_id, value)
select floor(random() * 100), random()
from generate_series(0, 1000000) as g (i);

vacuum analyze data;

explain (analyze, settings, costs off, timing off, summary off)
with keys(id) as (
     values (1),(2)
) select * from data join keys on (data.key_id = keys.id);
                                  QUERY PLAN
----------------------------------------------------------------------------
  Gather (actual rows=19845 loops=1)
    Workers Planned: 2
    Workers Launched: 2
    ->  Hash Join (actual rows=6615 loops=3)
          Hash Cond: (data_2.key_id = "*VALUES*".column1)
          ->  Parallel Append (actual rows=333334 loops=3)
                ->  Parallel Seq Scan on data_2 (actual rows=126670 loops=3)
                ->  Parallel Seq Scan on data_1 (actual rows=160458 loops=2)
                ->  Parallel Seq Scan on data_0 (actual rows=299075 loops=1)
          ->  Hash (actual rows=2 loops=3)
                Buckets: 1024  Batches: 1  Memory Usage: 5kB
                ->  Values Scan on "*VALUES*" (actual rows=2 loops=3)

We see that all partitions scanned. But after creating index postgres 
actually looks only to selected partitions:

create index on data(key_id);

explain (analyze, settings, costs off, timing off, summary off)
with keys(id) as (
     values (1),(2)
) select * from data join keys on (data.key_id = keys.id);
                                      QUERY PLAN
-------------------------------------------------------------------------------------
  Nested Loop (actual rows=19845 loops=1)
    ->  Values Scan on "*VALUES*" (actual rows=2 loops=1)

    ->  Append (actual rows=9922 loops=2)
          ->  Bitmap Heap Scan on data_0 (actual rows=9926 loops=1)
                Recheck Cond: (key_id = "*VALUES*".column1)
                Heap Blocks: exact=1324
                ->  Bitmap Index Scan on data_0_key_id_idx (actual 
rows=9926 loops=1)
                      Index Cond: (key_id = "*VALUES*".column1)
          ->  Bitmap Heap Scan on data_1 (never executed)
                Recheck Cond: (key_id = "*VALUES*".column1)
                ->  Bitmap Index Scan on data_1_key_id_idx (never executed)
                      Index Cond: (key_id = "*VALUES*".column1)
          ->  Bitmap Heap Scan on data_2 (actual rows=9919 loops=1)
                Recheck Cond: (key_id = "*VALUES*".column1)
                Heap Blocks: exact=1679
                ->  Bitmap Index Scan on data_2_key_id_idx (actual 
rows=9919 loops=1)
                      Index Cond: (key_id = "*VALUES*".column1)

Why runtime partition pruning needs index? Is it intended behavior?

-- 

-----
Pavel Luzanov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company