pg_upgrade: Retrieve subscription count more efficiently.

Presently, pg_upgrade obtains the number of subscriptions in the
to-be-upgraded cluster by first querying pg_subscription in every
database for the number of subscriptions in only that database.
Then, in count_old_cluster_subscriptions(), it adds all the values
collected in the first step.  This is expensive, especially when
there are many databases.

Fortunately, there is a better way to retrieve the subscription
count.  Since pg_subscription is a shared catalog, we only need to
connect to a single database and query it once.  This commit
modifies pg_upgrade to use that approach, which also allows us to
trim several lines of code.  In passing, move the call to
get_db_subscription_count(), which has been renamed to
get_subscription_count(), from get_db_rel_and_slot_infos() to the
dedicated >= v17 section in check_and_dump_old_cluster().

We may be able to make similar improvements to
get_old_cluster_logical_slot_infos(), but that is left as a future
exercise.

Reviewed-by: Michael Paquier, Amit Kapila
Discussion: https://postgr.es/m/ZprQJv_TxccN3tkr%40nathan
Backpatch-through: 17

src/bin/pg_upgrade/check.c

@@ -609,8 +609,10 @@ check_and_dump_old_cluster(bool live_check)
 
 		/*
 		 * Subscriptions and their dependencies can be migrated since PG17.
-		 * See comments atop get_db_subscription_count().
+		 * Before that the logical slots are not upgraded, so we will not be
+		 * able to upgrade the logical replication clusters completely.
 		 */
+		get_subscription_count(&old_cluster);
 		check_old_cluster_subscription_state();
 	}
 
@@ -1797,17 +1799,14 @@ check_new_cluster_subscription_configuration(void)
 {
 	PGresult   *res;
 	PGconn	   *conn;
-	int			nsubs_on_old;
 	int			max_replication_slots;
 
 	/* Subscriptions and their dependencies can be migrated since PG17. */
 	if (GET_MAJOR_VERSION(old_cluster.major_version) < 1700)
 		return;
 
-	nsubs_on_old = count_old_cluster_subscriptions();
-
 	/* Quick return if there are no subscriptions to be migrated. */
-	if (nsubs_on_old == 0)
+	if (old_cluster.nsubs == 0)
 		return;
 
 	prep_status("Checking for new cluster configuration for subscriptions");
@@ -1821,10 +1820,10 @@ check_new_cluster_subscription_configuration(void)
 		pg_fatal("could not determine parameter settings on new cluster");
 
 	max_replication_slots = atoi(PQgetvalue(res, 0, 0));
-	if (nsubs_on_old > max_replication_slots)
+	if (old_cluster.nsubs > max_replication_slots)
 		pg_fatal("\"max_replication_slots\" (%d) must be greater than or equal to the number of "
 				 "subscriptions (%d) on the old cluster",
-				 max_replication_slots, nsubs_on_old);
+				 max_replication_slots, old_cluster.nsubs);
 
 	PQclear(res);
 	PQfinish(conn);

src/bin/pg_upgrade/info.c

@@ -28,7 +28,6 @@ static void print_db_infos(DbInfoArr *db_arr);
 static void print_rel_infos(RelInfoArr *rel_arr);
 static void print_slot_infos(LogicalSlotInfoArr *slot_arr);
 static void get_old_cluster_logical_slot_infos(DbInfo *dbinfo, bool live_check);
-static void get_db_subscription_count(DbInfo *dbinfo);
 
 
 /*
@@ -293,15 +292,8 @@ get_db_rel_and_slot_infos(ClusterInfo *cluster, bool live_check)
 
 		get_rel_infos(cluster, pDbInfo);
 
-		/*
-		 * Retrieve the logical replication slots infos and the subscriptions
-		 * count for the old cluster.
-		 */
 		if (cluster == &old_cluster)
-		{
 			get_old_cluster_logical_slot_infos(pDbInfo, live_check);
-			get_db_subscription_count(pDbInfo);
-		}
 	}
 
 	if (cluster == &old_cluster)
@@ -748,54 +740,25 @@ count_old_cluster_logical_slots(void)
 }
 
 /*
- * get_db_subscription_count()
+ * get_subscription_count()
  *
- * Gets the number of subscriptions in the database referred to by "dbinfo".
+ * Gets the number of subscriptions in the cluster.
- *
- * Note: This function will not do anything if the old cluster is pre-PG17.
- * This is because before that the logical slots are not upgraded, so we will
- * not be able to upgrade the logical replication clusters completely.
  */
-static void
+void
-get_db_subscription_count(DbInfo *dbinfo)
+get_subscription_count(ClusterInfo *cluster)
 {
 	PGconn	   *conn;
 	PGresult   *res;
 
-	/* Subscriptions can be migrated since PG17. */
+	conn = connectToServer(cluster, "template1");
-	if (GET_MAJOR_VERSION(old_cluster.major_version) < 1700)
-		return;
-
-	conn = connectToServer(&old_cluster, dbinfo->db_name);
 	res = executeQueryOrDie(conn, "SELECT count(*) "
-							"FROM pg_catalog.pg_subscription WHERE subdbid = %u",
+							"FROM pg_catalog.pg_subscription");
-							dbinfo->db_oid);
+	cluster->nsubs = atoi(PQgetvalue(res, 0, 0));
-	dbinfo->nsubs = atoi(PQgetvalue(res, 0, 0));
 
 	PQclear(res);
 	PQfinish(conn);
 }
 
-/*
- * count_old_cluster_subscriptions()
- *
- * Returns the number of subscriptions for all databases.
- *
- * Note: this function always returns 0 if the old_cluster is PG16 and prior
- * because we gather subscriptions only for cluster versions greater than or
- * equal to PG17. See get_db_subscription_count().
- */
-int
-count_old_cluster_subscriptions(void)
-{
-	int			nsubs = 0;
-
-	for (int dbnum = 0; dbnum < old_cluster.dbarr.ndbs; dbnum++)
-		nsubs += old_cluster.dbarr.dbs[dbnum].nsubs;
-
-	return nsubs;
-}
-
 static void
 free_db_and_rel_infos(DbInfoArr *db_arr)
 {

src/bin/pg_upgrade/pg_upgrade.h

@@ -197,7 +197,6 @@ typedef struct
 											 * path */
 	RelInfoArr	rel_arr;		/* array of all user relinfos */
 	LogicalSlotInfoArr slot_arr;	/* array of all LogicalSlotInfo */
-	int			nsubs;			/* number of subscriptions */
 } DbInfo;
 
 /*
@@ -296,6 +295,7 @@ typedef struct
 	char		major_version_str[64];	/* string PG_VERSION of cluster */
 	uint32		bin_version;	/* version returned from pg_ctl */
 	const char *tablespace_suffix;	/* directory specification */
+	int			nsubs;			/* number of subscriptions */
 } ClusterInfo;
 
 
@@ -430,7 +430,7 @@ FileNameMap *gen_db_file_maps(DbInfo *old_db,
 							  const char *new_pgdata);
 void		get_db_rel_and_slot_infos(ClusterInfo *cluster, bool live_check);
 int			count_old_cluster_logical_slots(void);
-int			count_old_cluster_subscriptions(void);
+void		get_subscription_count(ClusterInfo *cluster);
 
 /* option.c */