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

1 files changed, 2 insertions, 2 deletions

diff --git a/src/bin/pg_upgrade/pg_upgrade.h b/src/bin/pg_upgrade/pg_upgrade.h
index 8afe240bdfb..e2b99b49fab 100644
--- a/src/bin/pg_upgrade/pg_upgrade.h
+++ b/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 */