Understanding explain costs - Mailing list pgsql-general

Hi,
Trying to understand the planner estimate costs  ... one index scan
seems to be much more expensive then another.  Here are the facts:

  tiger=# \dbk_inv
             Table "bk_inv"
   Attribute |     Type     | Modifier
  -----------+--------------+----------
   store     | varchar(5)   |
   isbn      | varchar(10)  | not null
   qty       | numeric(5,0) |
   week      | numeric(6,0) |
  Indices: bk_inv_isbn_idx,
           bk_inv_store_idx

  tiger=# \dbk_inv_isbn_idx
   Index "bk_inv_isbn_idx"
   Attribute |    Type
  -----------+-------------
   isbn      | varchar(10)
  btree

  tiger=# \dbk_inv_store_idx
  Index "bk_inv_store_idx"
   Attribute |    Type
  -----------+------------
   store     | varchar(5)
  btree

There are about 50,000,000 rows and you can see a full scan on the table
is expensive:

  tiger=# explain select * from bk_inv;
  NOTICE:  QUERY PLAN:

  Seq Scan on bk_inv  (cost=0.00..999623.77 rows=46790877 width=48)


However using the isbn index costs are much less:

  tiger=# explain select * from bk_inv where isbn = 'foo';
  NOTICE:  QUERY PLAN:

  Index Scan using bk_inv_isbn_idx on bk_inv  (cost=0.00..53.13
  rows=13 width=48)


However, however using the store index costs are still rather high  -
why is that!!??

  tiger=# explain select * from bk_inv where store = 'foo';
  NOTICE:  QUERY PLAN:

  Index Scan using bk_inv_store_idx on bk_inv  (cost=0.00..53456.09
  rows=13488 width=48)

Incidently the store index is slightly smaller than the isbn index ...

[postgres@roma tiger]$ ls -l bk_inv*
-rw-------    1 postgres postgres 1073741824 Oct 10 14:28 bk_inv
-rw-------    1 postgres postgres 1073741824 Oct 10 10:15 bk_inv.1
-rw-------    1 postgres postgres 1073741824 Oct 10 10:17 bk_inv.2
-rw-------    1 postgres postgres 1073741824 Oct 10 10:19 bk_inv.3
-rw-------    1 postgres postgres 60841984 Oct 11 15:51 bk_inv.4
-rw-------    1 postgres postgres 1073741824 Oct 10 13:37
bk_inv_isbn_idx
-rw-------    1 postgres postgres 566288384 Oct 10 14:31
bk_inv_isbn_idx.1
-rw-------    1 postgres postgres 1073741824 Oct 11 13:13
bk_inv_store_idx
-rw-------    1 postgres postgres 65921024 Oct 11 13:13
bk_inv_store_idx.1


Am I missing certain fundamentals about the planner/executor?

Thanks, David Link