pgstattuple: Fix failure with pgstathashindex() for partitioned indexes

As coded, the function relied on index_open() when opening an index
relation, allowing partitioned indexes to be processed by
pgstathashindex().  This was leading to a "could not open file" error
because partitioned indexes have no physical files, or to a crash with
an assertion failure (like on HEAD).

This issue is fixed by applying the same checks as the other stat
functions for indexes, with a lookup at both RELKIND_INDEX and the index
AM expected.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/18246-f4d9ff7cb3af77e6@postgresql.org
Backpatch-through: 12

contrib/pgstattuple/expected/pgstattuple.out

@@ -153,6 +153,7 @@ ERROR:  relation "test_hashidx" is not a GIN index
 -- check that using any of these functions with unsupported relations will fail
 create table test_partitioned (a int) partition by range (a);
 create index test_partitioned_index on test_partitioned(a);
+create index test_partitioned_hash_index on test_partitioned using hash(a);
 -- these should all fail
 select pgstattuple('test_partitioned');
 ERROR:  "test_partitioned" (partitioned table) is not supported
@@ -167,7 +168,9 @@ ERROR:  relation "test_partitioned" is not a btree index
 select pgstatginindex('test_partitioned');
 ERROR:  relation "test_partitioned" is not a GIN index
 select pgstathashindex('test_partitioned');
-ERROR:  "test_partitioned" is not an index
+ERROR:  relation "test_partitioned" is not a hash index
+select pgstathashindex('test_partitioned_hash_index');
+ERROR:  relation "test_partitioned_hash_index" is not a hash index
 create view test_view as select 1;
 -- these should all fail
 select pgstattuple('test_view');
@@ -181,7 +184,7 @@ ERROR:  relation "test_view" is not a btree index
 select pgstatginindex('test_view');
 ERROR:  relation "test_view" is not a GIN index
 select pgstathashindex('test_view');
-ERROR:  "test_view" is not an index
+ERROR:  relation "test_view" is not a hash index
 create foreign data wrapper dummy;
 create server dummy_server foreign data wrapper dummy;
 create foreign table test_foreign_table () server dummy_server;
@@ -197,7 +200,7 @@ ERROR:  relation "test_foreign_table" is not a btree index
 select pgstatginindex('test_foreign_table');
 ERROR:  relation "test_foreign_table" is not a GIN index
 select pgstathashindex('test_foreign_table');
-ERROR:  "test_foreign_table" is not an index
+ERROR:  relation "test_foreign_table" is not a hash index
 -- a partition of a partitioned table should work though
 create table test_partition partition of test_partitioned for values from (1) to (100);
 select pgstattuple('test_partition');
@@ -224,7 +227,7 @@ ERROR:  relation "test_partition" is not a btree index
 select pgstatginindex('test_partition');
 ERROR:  relation "test_partition" is not a GIN index
 select pgstathashindex('test_partition');
-ERROR:  "test_partition" is not an index
+ERROR:  relation "test_partition" is not a hash index
 -- 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);

contrib/pgstattuple/pgstatindex.c

@@ -615,10 +615,9 @@ pgstathashindex(PG_FUNCTION_ARGS)
 	float8		free_percent;
 	uint64		total_space;
 
-	rel = index_open(relid, AccessShareLock);
+	rel = relation_open(relid, AccessShareLock);
 
-	/* index_open() checks that it's an index */
-	if (!IS_HASH(rel))
+	if (!IS_INDEX(rel) || !IS_HASH(rel))
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
 				 errmsg("relation \"%s\" is not a hash index",

contrib/pgstattuple/sql/pgstattuple.sql

@@ -65,6 +65,7 @@ select pgstatginindex('test_hashidx');
 -- check that using any of these functions with unsupported relations will fail
 create table test_partitioned (a int) partition by range (a);
 create index test_partitioned_index on test_partitioned(a);
+create index test_partitioned_hash_index on test_partitioned using hash(a);
 -- these should all fail
 select pgstattuple('test_partitioned');
 select pgstattuple('test_partitioned_index');
@@ -73,6 +74,7 @@ select pg_relpages('test_partitioned');
 select pgstatindex('test_partitioned');
 select pgstatginindex('test_partitioned');
 select pgstathashindex('test_partitioned');
+select pgstathashindex('test_partitioned_hash_index');
 
 create view test_view as select 1;
 -- these should all fail