[GENERAL] Why does index not use for CTE query?

[AI Rumman]

Why does index not use for CTE query?
I am using Postgresql 9.1

select version();

version
--------------------------------------------------------------------------------------------------------
 PostgreSQL 9.1.1 on i686-pc-linux-gnu, compiled by gcc (GCC) 4.1.2
20080704 (Red Hat 4.1.2-50), 32-bit
(1 row)


\d t1
      Table "public.t1"
 Column |  Type   | Modifiers
--------+---------+-----------
 i      | integer |
 nam    | text    |
Indexes:
    "t1_i_idx" btree (i)


analyze t1;
explain select * from t1 where  i=2;
                    QUERY PLAN
--------------------------------------------------
 Seq Scan on t1  (cost=0.00..1.09 rows=4 width=9)
   Filter: (i = 2)
(2 rows)

set enable_seqscan = off;

explain select * from t1 where  i=2;
                             QUERY PLAN
--------------------------------------------------------------------
 Index Scan using t1_i_idx on t1  (cost=0.00..12.32 rows=4 width=9)
   Index Cond: (i = 2)
(2 rows)


explain
with q as (select * from t1)
select * from q where  i=2;

select * from q where  i=2;
                                  QUERY
PLAN
------------------------------------------------------------------------------
 CTE Scan on q  (cost=10000000001.07..10000000001.23 rows=1 width=36)
   Filter: (i = 2)
   CTE q
     ->  Seq Scan on t1  (cost=10000000000.00..10000000001.07 rows=7
width=9)
(4 rows)

Index is not using here.
Could you tell me why?