PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> postgres=# grant EXECUTE on pg_catalog.pg_start_backup to backup ;
> ERROR: relation "pg_catalog.pg_start_backup" does not exist
pg_start_backup is a function not a table. Try
grant execute on function pg_start_backup to backup;
(In versions before PG10 you'd need to specify the function's
arguments too. Newer versions allow this shortcut as long as there's
only one function by that name.)
regards, tom lane