diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index aa67f75c0c..2fba6b772d 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -848,18 +848,6 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 			strcpy(out_dbname, dbname);
 	}
 
-	/* Now we can mark our PGPROC entry with the database ID */
-	/* (We assume this is an atomic store so no lock is needed) */
-	MyProc->databaseId = MyDatabaseId;
-
-	/*
-	 * We established a catalog snapshot while reading pg_authid and/or
-	 * pg_database; but until we have set up MyDatabaseId, we won't react to
-	 * incoming sinval messages for unshared catalogs, so we won't realize it
-	 * if the snapshot has been invalidated.  Assume it's no good anymore.
-	 */
-	InvalidateCatalogSnapshot();
-
 	/*
 	 * Now, take a writer's lock on the database we are trying to connect to.
 	 * If there is a concurrently running DROP DATABASE on that database, this
@@ -867,9 +855,13 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	 * pg_database).
 	 *
 	 * Note that the lock is not held long, only until the end of this startup
-	 * transaction.  This is OK since we are already advertising our use of
-	 * the database in the PGPROC array; anyone trying a DROP DATABASE after
-	 * this point will see us there.
+	 * transaction.  This is OK since we will advertise our use of the
+	 * database in the ProcArray before dropping the lock (in fact, that's the
+	 * next thing to do).  Anyone trying a DROP DATABASE after this point will
+	 * see us in the array once they have the lock.  Ordering is important for
+	 * this because we don't want to advertise ourselves as being in this
+	 * database until we have the lock; otherwise we create what amounts to a
+	 * deadlock with CountOtherDBBackends().
 	 *
 	 * Note: use of RowExclusiveLock here is reasonable because we envision
 	 * our session as being a concurrent writer of the database.  If we had a
@@ -881,6 +873,28 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 		LockSharedObject(DatabaseRelationId, MyDatabaseId, 0,
 						 RowExclusiveLock);
 
+	/*
+	 * Now we can mark our PGPROC entry with the database ID.
+	 *
+	 * We assume this is an atomic store so no lock is needed; though actually
+	 * things would work fine even if it weren't atomic.  Anyone searching the
+	 * ProcArray for this database's ID should hold the database lock, so they
+	 * would not be executing concurrently with this store.  A process looking
+	 * for another database's ID could in theory see a chance match if it read
+	 * a partially-updated databaseId value; but as long as all such searches
+	 * wait and retry, as in CountOtherDBBackends(), they will certainly see
+	 * the correct value on their next try.
+	 */
+	MyProc->databaseId = MyDatabaseId;
+
+	/*
+	 * We established a catalog snapshot while reading pg_authid and/or
+	 * pg_database; but until we have set up MyDatabaseId, we won't react to
+	 * incoming sinval messages for unshared catalogs, so we won't realize it
+	 * if the snapshot has been invalidated.  Assume it's no good anymore.
+	 */
+	InvalidateCatalogSnapshot();
+
 	/*
 	 * Recheck pg_database to make sure the target database hasn't gone away.
 	 * If there was a concurrent DROP DATABASE, this ensures we will die