How is execution plan cost calculated for index scan

Hi all:

I want to see the explain plan for a simple query. My question is : How
is the cost calculated?

The cost parameter is:

random_page_cost = 4

seq_page_cost = 1

cpu_tuple_cost =0.01

cpu_operator_cost =0.0025

And the table and its index physical situation are as following:

postgres=# select relpages, reltuples from pg_class where relname =
'pg_proc';

relpages | reltuples

----------+-----------

62 |
2490

postgres=# select relpages, reltuples from pg_class where relname =
'pg_proc_oid_index';

relpages | reltuples

----------+-----------

9 |
2490

The explain plan is:

postgres=# explain SELECT * FROM pg_proc where
oid=1;

QUERY
PLAN

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

Index Scan using pg_proc_oid_index on pg_proc (cost=0.00..8.27 rows=1
width=548)

Index Cond: (oid =
1::oid)

(2
rows)

I think in the worst situation ,

Firstly, database need to search for 9 index pages by sequential to find
the index entry. For each index page in memory, every “index tuple” need
to be scanned.

Then , using the key entry, it need to make a random page read for the
real data into memory, then scan the data tuple is scanned until the reall
one is found

(or just directly locate to the data block after read the data page into
memory )

So at least the evaluated max cost should be bigger than 9 index pages *
seq_page_cost , so it should be bigger than 9. Here I haven't added the
random page read cost for data.

But what I got is max is 8.27. How is the result of 8.27 be calculated?

Furthermore, I tried to find the logic in source code, I think it might be
costsize.c in src/backend/optimizer/, by debugging it, I found that:

When I use [ explain SELECT * FROM pg_proc where oid=1;] , I can found
that cost_index function is called.

The result returned for path->path.total_cost is 86698968. And
86698968/1024/1024 = 82.68258 . If devided by 10 , is near 8.27. but this
is still a little odd.

In the above case, can I say that the cost formula for index scan is
in-- the cost_index function ?

Thanks in advance