Add infrastructure for mapping relfilenodes to relation OIDs.

Future patches are expected to introduce logical replication that
works by decoding WAL.  WAL contains relfilenodes rather than relation
OIDs, so this infrastructure will be needed to find the relation OID
based on WAL contents.

If logical replication does not make it into this release, we probably
should consider reverting this, since it will add some overhead to DDL
operations that create new relations.  One additional index insert per
pg_class row is not a large overhead, but it's more than zero.
Another way of meeting the needs of logical replication would be to
the relation OID to WAL, but that would burden DML operations, not
only DDL.

Andres Freund, with some changes by me.  Design review, in earlier
versions, by Álvaro Herrera.

1 files changed, 28 insertions, 0 deletions

diff --git a/src/backend/utils/adt/dbsize.c b/src/backend/utils/adt/dbsize.c
index 34482abee3e..21d1c946abe 100644
--- a/src/backend/utils/adt/dbsize.c
+++ b/src/backend/utils/adt/dbsize.c
@@ -28,6 +28,7 @@
 #include "utils/builtins.h"
 #include "utils/numeric.h"
 #include "utils/rel.h"
+#include "utils/relfilenodemap.h"
 #include "utils/relmapper.h"
 #include "utils/syscache.h"
 
@@ -756,6 +757,33 @@ pg_relation_filenode(PG_FUNCTION_ARGS)
 }
 
 /*
+ * Get the relation via (reltablespace, relfilenode)
+ *
+ * This is expected to be used when somebody wants to match an individual file
+ * on the filesystem back to its table. Thats not trivially possible via
+ * pg_class because that doesn't contain the relfilenodes of shared and nailed
+ * tables.
+ *
+ * We don't fail but return NULL if we cannot find a mapping.
+ *
+ * Instead of knowing DEFAULTTABLESPACE_OID you can pass 0.
+ */
+Datum
+pg_filenode_relation(PG_FUNCTION_ARGS)
+{
+	Oid			reltablespace = PG_GETARG_OID(0);
+	Oid			relfilenode = PG_GETARG_OID(1);
+	Oid			heaprel = InvalidOid;
+
+	heaprel = RelidByRelfilenode(reltablespace, relfilenode);
+
+	if (!OidIsValid(heaprel))
+		PG_RETURN_NULL();
+	else
+		PG_RETURN_OID(heaprel);
+}
+
+/*
  * Get the pathname (relative to $PGDATA) of a relation
  *
  * See comments for pg_relation_filenode.