how does the planer to estimate row when i use order by and group by

(

  id serial NOT NULL,

  name character varying(20),

  modifytime timestamp without time zone,

  CONSTRAINT a_pk PRIMARY KEY (id)

)

explain  analyze

select * from 

( select id from a order by id ) d 

group by  id;

"Group  (cost=0.15..66.42 rows=200 width=4) (actual time=0.008..0.008 rows=0 loops=1)"

"  ->  Index Only Scan using a_pk on a  (cost=0.15..56.30 rows=810 width=4) (actual time=0.006..0.006 rows=0 loops=1)"

"        Heap Fetches: 0"

"Total runtime: 0.046 ms"