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, 53 insertions, 0 deletions

diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
index 2c7d9f3287b..18f0342a7de 100644
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -181,6 +181,59 @@ RelationMapOidToFilenode(Oid relationId, bool shared)
 }
 
 /*
+ * RelationMapFilenodeToOid
+ *
+ * Do the reverse of the normal direction of mapping done in
+ * RelationMapOidToFilenode.
+ *
+ * This is not supposed to be used during normal running but rather for
+ * information purposes when looking at the filesystem or xlog.
+ *
+ * Returns InvalidOid if the OID is not known; this can easily happen if the
+ * relfilenode doesn't pertain to a mapped relation.
+ */
+Oid
+RelationMapFilenodeToOid(Oid filenode, bool shared)
+{
+	const RelMapFile *map;
+	int32		i;
+
+	/* If there are active updates, believe those over the main maps */
+	if (shared)
+	{
+		map = &active_shared_updates;
+		for (i = 0; i < map->num_mappings; i++)
+		{
+			if (filenode == map->mappings[i].mapfilenode)
+				return map->mappings[i].mapoid;
+		}
+		map = &shared_map;
+		for (i = 0; i < map->num_mappings; i++)
+		{
+			if (filenode == map->mappings[i].mapfilenode)
+				return map->mappings[i].mapoid;
+		}
+	}
+	else
+	{
+		map = &active_local_updates;
+		for (i = 0; i < map->num_mappings; i++)
+		{
+			if (filenode == map->mappings[i].mapfilenode)
+				return map->mappings[i].mapoid;
+		}
+		map = &local_map;
+		for (i = 0; i < map->num_mappings; i++)
+		{
+			if (filenode == map->mappings[i].mapfilenode)
+				return map->mappings[i].mapoid;
+		}
+	}
+
+	return InvalidOid;
+}
+
+/*
  * RelationMapUpdateMap
  *
  * Install a new relfilenode mapping for the specified relation.