sequence scan on PK

Hi,


I understand that when a table contains only a few rows it is better to
do a sequence scan than an index scan. But is this also for a table with
99 records?

A table contains
id                                          integer (primary key)
name                                    varchar(70)
parent                                   integer
comment                              text
owner                                  integer
inheritAccess                       integer
defaultAccess                      integer
sequence                              bigint
contentsinheritaccessmove   integer
contentsinheritaccessadd      integer


explain select * from tblFolders where id=90;
                        QUERY PLAN
-----------------------------------------------------------
 Seq Scan on tblfolders  (cost=0.00..3.24 rows=1 width=50)
   Filter: (id = 90)


(I have analyze table bit still a sequence scan).

With how manys rows it is  ok to do an index scan or sequence scan? How
is this calculated in pg?

Regards
 Jer