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diff --git a/src/include/storage/relfilenode.h b/src/include/storage/relfilenode.h
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+/*-------------------------------------------------------------------------
+ *
+ * relfilenode.h
+ *	  Physical access information for relations.
+ *
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/relfilenode.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef RELFILENODE_H
+#define RELFILENODE_H
+
+#include "common/relpath.h"
+#include "storage/backendid.h"
+
+/*
+ * RelFileNode must provide all that we need to know to physically access
+ * a relation, with the exception of the backend ID, which can be provided
+ * separately. Note, however, that a "physical" relation is comprised of
+ * multiple files on the filesystem, as each fork is stored as a separate
+ * file, and each fork can be divided into multiple segments. See md.c.
+ *
+ * spcNode identifies the tablespace of the relation.  It corresponds to
+ * pg_tablespace.oid.
+ *
+ * dbNode identifies the database of the relation.  It is zero for
+ * "shared" relations (those common to all databases of a cluster).
+ * Nonzero dbNode values correspond to pg_database.oid.
+ *
+ * relNode identifies the specific relation.  relNode corresponds to
+ * pg_class.relfilenode (NOT pg_class.oid, because we need to be able
+ * to assign new physical files to relations in some situations).
+ * Notice that relNode is only unique within a database in a particular
+ * tablespace.
+ *
+ * Note: spcNode must be GLOBALTABLESPACE_OID if and only if dbNode is
+ * zero.  We support shared relations only in the "global" tablespace.
+ *
+ * Note: in pg_class we allow reltablespace == 0 to denote that the
+ * relation is stored in its database's "default" tablespace (as
+ * identified by pg_database.dattablespace).  However this shorthand
+ * is NOT allowed in RelFileNode structs --- the real tablespace ID
+ * must be supplied when setting spcNode.
+ *
+ * Note: in pg_class, relfilenode can be zero to denote that the relation
+ * is a "mapped" relation, whose current true filenode number is available
+ * from relmapper.c.  Again, this case is NOT allowed in RelFileNodes.
+ *
+ * Note: various places use RelFileNode in hashtable keys.  Therefore,
+ * there *must not* be any unused padding bytes in this struct.  That
+ * should be safe as long as all the fields are of type Oid.
+ */
+typedef struct RelFileNode
+{
+	Oid			spcNode;		/* tablespace */
+	Oid			dbNode;			/* database */
+	Oid			relNode;		/* relation */
+} RelFileNode;
+
+/*
+ * Augmenting a relfilenode with the backend ID provides all the information
+ * we need to locate the physical storage.  The backend ID is InvalidBackendId
+ * for regular relations (those accessible to more than one backend), or the
+ * owning backend's ID for backend-local relations.  Backend-local relations
+ * are always transient and removed in case of a database crash; they are
+ * never WAL-logged or fsync'd.
+ */
+typedef struct RelFileNodeBackend
+{
+	RelFileNode node;
+	BackendId	backend;
+} RelFileNodeBackend;
+
+#define RelFileNodeBackendIsTemp(rnode) \
+	((rnode).backend != InvalidBackendId)
+
+/*
+ * Note: RelFileNodeEquals and RelFileNodeBackendEquals compare relNode first
+ * since that is most likely to be different in two unequal RelFileNodes.  It
+ * is probably redundant to compare spcNode if the other fields are found equal,
+ * but do it anyway to be sure.  Likewise for checking the backend ID in
+ * RelFileNodeBackendEquals.
+ */
+#define RelFileNodeEquals(node1, node2) \
+	((node1).relNode == (node2).relNode && \
+	 (node1).dbNode == (node2).dbNode && \
+	 (node1).spcNode == (node2).spcNode)
+
+#define RelFileNodeBackendEquals(node1, node2) \
+	((node1).node.relNode == (node2).node.relNode && \
+	 (node1).node.dbNode == (node2).node.dbNode && \
+	 (node1).backend == (node2).backend && \
+	 (node1).node.spcNode == (node2).node.spcNode)
+
+#endif							/* RELFILENODE_H */