Use order by clause, got index scan involved

Hi all:

What confused me is that:  When I select data using order by  clause, I got the following execution plan:

postgres=# set session enable_indexscan=true;                                                                                              

SET                                                                                                                                        

postgres=# explain SELECT * FROM pg_proc ORDER BY oid;                                                                                         

                                       QUERY PLAN                                                                                                                              

----------------------------------------------------------------------------------------                                                                                            

 Index Scan using pg_proc_oid_index on pg_proc  (cost=0.00..321.60 rows=2490 width=552)                                                                                                                                      

(1 row)                                                                                                                                

postgres=#

My Question is :

 If I want to find record using the where clause which hold the id column,  the index scan might be used.

But  I just want to get all the  records on sorted output format,  Why index scan can be used here?

I can’t imagine  that:

Step 1 Index is read into memory, then for each tuple in it,

Step 2 Then we got  the address of  related data block, and then access the data block .

Step 2 will be repeated for many times. I think it is not efficient.

But comparing with sort , I got that  even index scan with all the entry , the cost is still lower than sort operation:

That is to say: cost of seq scan + sort   > cost of  index scan for every index entry  + cost of access for every related data ?

Maybe the database system is clever enough to  accumulate data access for same physical page, and  reduce the times of physical page acess ?

And can somebody kindly give  some more detailed information which help to know the execution plan calculation process?

Thanks in advance.