sequencial scans

Greetings,

With short words (time is money and my english is very bad), why sequencial scans in the example below ?

persona=# CREATE TABLE teste_erro(
persona(# cod integer,
persona(# CONSTRAINT XPKteste_erro PRIMARY KEY(cod));
NOTICE: CREATE TABLE/PRIMARY KEY will create implicit index 'xpkteste_erro' for table 'teste_erro'
CREATE

persona=# CREATE FUNCTION teste_proc(VARCHAR, VARCHAR) RETURNS integer AS '
persona'# DECLARE
persona'# numeroa ALIAS for $1;
persona'# dddlocal ALIAS for $2;
persona'# BEGIN
persona'# RETURN 666;
persona'# END;
persona'# ' LANGUAGE 'plpgsql';
CREATE

persona=# explain select * from teste_erro where cod = teste_proc('3138414411','19');
NOTICE: QUERY PLAN:
Seq Scan on teste_erro (cost=0.00..25.00 rows=10 width=4)
EXPLAIN

persona=#explain select * from teste_erro where cod = 313;
NOTICE: QUERY PLAN:
Index Scan using xpkteste_erro on teste_erro (cost=0.00..8.14 rows=10 width=4)
EXPLAIN

Regards,

José Vilson de Mello de Farias
Dígitro Tecnologia Ltda - Brazil