Cache table AM in Chunk struct

A chunk can use different table access methods so to support this more
easily, cache the AM oid in the chunk struct. This allows identifying
the access method quickly at, e.g., planning time.

src/chunk.c

@@ -1620,6 +1620,7 @@ chunk_tuple_found(TupleInfo *ti, void *arg)
 	chunk->hypertable_relid = ts_hypertable_id_to_relid(chunk->fd.hypertable_id, false);
 
 	chunk->relkind = get_rel_relkind(chunk->table_id);
+	chunk->amoid = ts_get_rel_am(chunk->table_id);
 
 	Ensure(chunk->relkind > 0,
 		   "relkind for chunk \"%s\".\"%s\" is invalid",

src/chunk.h

@@ -66,6 +66,7 @@ typedef struct Chunk
 	char relkind;
 	Oid table_id;
 	Oid hypertable_relid;
+	Oid amoid; /* Table access method used by chunk */
 
 	/*
 	 * The hypercube defines the chunks position in the N-dimensional space.

src/utils.c

@@ -1748,3 +1748,25 @@ ts_update_placeholder(PG_FUNCTION_ARGS)
 	elog(ERROR, "this stub function is used only as placeholder during extension updates");
 	PG_RETURN_NULL();
 }
+
+/*
+ * Get the table access method Oid for a relation.
+ */
+Oid
+ts_get_rel_am(Oid relid)
+{
+	HeapTuple tuple;
+	Form_pg_class cform;
+	Oid amoid;
+
+	tuple = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
+
+	if (!HeapTupleIsValid(tuple))
+		elog(ERROR, "cache lookup failed for relation %u", relid);
+
+	cform = (Form_pg_class) GETSTRUCT(tuple);
+	amoid = cform->relam;
+	ReleaseSysCache(tuple);
+
+	return amoid;
+}

src/utils.h

@@ -371,3 +371,5 @@ ts_datum_set_objectid(const AttrNumber attno, NullableDatum *datums, const Oid v
 	else
 		datums[AttrNumberGetAttrOffset(attno)].isnull = true;
 }
+
+extern Oid ts_get_rel_am(Oid relid);