During pg_upgrade, conditionally skip transfer of FSMs.

If a heap on the old cluster has 4 pages or fewer, and the old cluster
was PG v11 or earlier, don't copy or link the FSM. This will shrink
space usage for installations with large numbers of small tables.

This will allow pg_upgrade to take advantage of commit b0eaa4c51b where
we have avoided creation of the free space map for small heap relations.

Author: John Naylor
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CACPNZCu4cOdm3uGnNEGXivy7Gz8UWyQjynDpdkPGabQ18_zK6g%40mail.gmail.com

doc/src/sgml/ref/pgupgrade.sgml

@@ -792,6 +792,13 @@ psql --username=postgres --file=script.sql postgres
    is down.
   </para>
 
+  <para>
+   In <productname>PostgreSQL</productname> 12 and later small tables by
+   default don't have a free space map, as a space optimization.  If you are
+   upgrading a pre-12 cluster, the free space maps of small tables will
+   likewise not be transferred to the new cluster.  
+  </para>
+
  </refsect1>
 
  <refsect1>

src/bin/pg_upgrade/info.c

@@ -200,6 +200,8 @@ create_rel_filename_map(const char *old_data, const char *new_data,
 
 	map->old_db_oid = old_db->db_oid;
 	map->new_db_oid = new_db->db_oid;
+	map->relpages = old_rel->relpages;
+	map->relkind = old_rel->relkind;
 
 	/*
 	 * old_relfilenode might differ from pg_class.oid (and hence
@@ -418,6 +420,7 @@ get_rel_infos(ClusterInfo *cluster, DbInfo *dbinfo)
 	char	   *nspname = NULL;
 	char	   *relname = NULL;
 	char	   *tablespace = NULL;
+	char	   *relkind = NULL;
 	int			i_spclocation,
 				i_nspname,
 				i_relname,
@@ -425,7 +428,9 @@ get_rel_infos(ClusterInfo *cluster, DbInfo *dbinfo)
 				i_indtable,
 				i_toastheap,
 				i_relfilenode,
-				i_reltablespace;
+				i_reltablespace,
+				i_relpages,
+				i_relkind;
 	char		query[QUERY_ALLOC];
 	char	   *last_namespace = NULL,
 			   *last_tablespace = NULL;
@@ -494,7 +499,7 @@ get_rel_infos(ClusterInfo *cluster, DbInfo *dbinfo)
 	 */
 	snprintf(query + strlen(query), sizeof(query) - strlen(query),
 			 "SELECT all_rels.*, n.nspname, c.relname, "
-			 "  c.relfilenode, c.reltablespace, %s "
+			 "  c.relfilenode, c.reltablespace, c.relpages, c.relkind, %s "
 			 "FROM (SELECT * FROM regular_heap "
 			 "      UNION ALL "
 			 "      SELECT * FROM toast_heap "
@@ -525,6 +530,8 @@ get_rel_infos(ClusterInfo *cluster, DbInfo *dbinfo)
 	i_relname = PQfnumber(res, "relname");
 	i_relfilenode = PQfnumber(res, "relfilenode");
 	i_reltablespace = PQfnumber(res, "reltablespace");
+	i_relpages = PQfnumber(res, "relpages");
+	i_relkind = PQfnumber(res, "relkind");
 	i_spclocation = PQfnumber(res, "spclocation");
 
 	for (relnum = 0; relnum < ntups; relnum++)
@@ -556,6 +563,11 @@ get_rel_infos(ClusterInfo *cluster, DbInfo *dbinfo)
 		curr->relname = pg_strdup(relname);
 
 		curr->relfilenode = atooid(PQgetvalue(res, relnum, i_relfilenode));
+		curr->relpages = atoi(PQgetvalue(res, relnum, i_relpages));
+
+		relkind = PQgetvalue(res, relnum, i_relkind);
+		curr->relkind = relkind[0];
+
 		curr->tblsp_alloc = false;
 
 		/* Is the tablespace oid non-default? */

src/bin/pg_upgrade/pg_upgrade.h

@@ -147,6 +147,8 @@ typedef struct
 	char	   *tablespace;		/* tablespace path; "" for cluster default */
 	bool		nsp_alloc;		/* should nspname be freed? */
 	bool		tblsp_alloc;	/* should tablespace be freed? */
+	int32		relpages;		/* # of pages -- see pg_class.h */
+	char		relkind;		/* relation kind -- see pg_class.h */
 } RelInfo;
 
 typedef struct
@@ -173,6 +175,10 @@ typedef struct
 	 */
 	Oid			old_relfilenode;
 	Oid			new_relfilenode;
+
+	int32		relpages;		/* # of pages -- see pg_class.h */
+	char		relkind;		/* relation kind -- see pg_class.h */
+
 	/* the rest are used only for logging and error reporting */
 	char	   *nspname;		/* namespaces */
 	char	   *relname;

src/bin/pg_upgrade/relfilenode.c

@@ -14,10 +14,12 @@
 #include <sys/stat.h>
 #include "catalog/pg_class_d.h"
 #include "access/transam.h"
+#include "storage/freespace.h"
 
 
 static void transfer_single_new_db(FileNameMap *maps, int size, char *old_tablespace);
 static void transfer_relfile(FileNameMap *map, const char *suffix, bool vm_must_add_frozenbit);
+static bool new_cluster_needs_fsm(FileNameMap *map);
 
 
 /*
@@ -174,6 +176,7 @@ transfer_single_new_db(FileNameMap *maps, int size, char *old_tablespace)
 				/*
 				 * Copy/link any fsm and vm files, if they exist
 				 */
+				if (new_cluster_needs_fsm(&maps[mapnum]))
 					transfer_relfile(&maps[mapnum], "_fsm", vm_must_add_frozenbit);
 				if (vm_crashsafe_match)
 					transfer_relfile(&maps[mapnum], "_vm", vm_must_add_frozenbit);
@@ -278,3 +281,61 @@ transfer_relfile(FileNameMap *map, const char *type_suffix, bool vm_must_add_fro
 			}
 	}
 }
+
+/*
+ * new_cluster_needs_fsm()
+ *
+ * Return false for small heaps if we're upgrading across PG 12, the first
+ * version where small heap relations don't have FSMs by default.
+ */
+static bool
+new_cluster_needs_fsm(FileNameMap *map)
+{
+	char		old_primary_file[MAXPGPATH];
+	struct stat statbuf;
+
+	/* fsm/vm files added in PG 8.4 */
+	Assert(GET_MAJOR_VERSION(old_cluster.major_version) >= 804);
+
+	if (!(GET_MAJOR_VERSION(old_cluster.major_version) <= 1100 &&
+		  GET_MAJOR_VERSION(new_cluster.major_version) >= 1200))
+		return true;
+
+	/* Always transfer FSMs of non-heap relations. */
+	if (map->relkind != RELKIND_RELATION &&
+		map->relkind != RELKIND_TOASTVALUE)
+		return true;
+
+	/*
+	 * If pg_class.relpages falsely reports that the heap is above the
+	 * threshold, we will transfer a FSM when we don't need to, but this is
+	 * harmless.
+	 */
+	if (map->relpages > HEAP_FSM_CREATION_THRESHOLD)
+		return true;
+
+	/* Determine path of the primary file. */
+	snprintf(old_primary_file, sizeof(old_primary_file), "%s%s/%u/%u",
+			 map->old_tablespace,
+			 map->old_tablespace_suffix,
+			 map->old_db_oid,
+			 map->old_relfilenode);
+
+	/*
+	 * If pg_class.relpages falsely reports that the heap is below the
+	 * threshold, a FSM would be skipped when we actually need it.  To guard
+	 * against this, we verify the size of the primary file.
+	 */
+	if (stat(old_primary_file, &statbuf) != 0)
+	{
+		pg_fatal("error while checking for file existence \"%s.%s\" (\"%s\"): %s\n",
+				 map->nspname, map->relname, old_primary_file, strerror(errno));
+
+		/* Keep compiler quiet. */
+		return false;
+	}
+	else if (statbuf.st_size > HEAP_FSM_CREATION_THRESHOLD * BLCKSZ)
+		return true;
+	else
+		return false;
+}