Pointed out by Michael Paquier
ERROR: relation "test_partition" is not a GIN index
select pgstathashindex('test_partition');
ERROR: "test_partition" is not an index
--- an actual index of a partitiond table should work though
+-- an actual index of a partitioned table should work though
create index test_partition_idx on test_partition(a);
create index test_partition_hash_idx on test_partition using hash (a);
WARNING: hash indexes are not WAL-logged and their use is discouraged
select pgstatginindex('test_partition');
select pgstathashindex('test_partition');
--- an actual index of a partitiond table should work though
+-- an actual index of a partitioned table should work though
create index test_partition_idx on test_partition(a);
create index test_partition_hash_idx on test_partition using hash (a);
-- these should work