Yet another question about not use on indexes

Hi,

I have created a table whit some indexes. I analize
the query of this table and never use index.

After this, I create a more simplistic table with two
columns and one index and the query uses the index.

Look at this:

pfc=# \d document
                                  Table
"public.document"
   Column   |           Type           |
      Modifiers
------------+--------------------------+----------------------------------------------------
 codi       | integer                  | not null
default nextval('seq_document'::text)
 nom        | character varying(32)    | not null
 descripcio | text                     |
 formulari  | integer                  |
 fitxer     | character varying(32)    |
 tamany     | integer                  | default -1
 data       | timestamp with time zone | default
('now'::text)::timestamp(6) with time zone
Indexes: document_pkey primary key btree (codi),
         ind_doc1 btree (codi),
         ind_document btree (formulari)
Triggers: RI_ConstraintTrigger_19414,
          RI_ConstraintTrigger_19418,
          RI_ConstraintTrigger_19419,
          actualitzaritemcercadocument,
          altaitemcercadocument,
          baixaitemcercadocument,
          eliminaracldocument,
          eliminaravaluaciodocument

pfc=# explain select * from document where codi=2;
                        QUERY PLAN
----------------------------------------------------------
 Seq Scan on document  (cost=0.00..1.19 rows=1
width=120)
   Filter: (codi = 2)
(2 rows)

This query must use index document_pkey but explain
tells us that the query does a Sequencial scan on
table document.

Look at this simplistic case:

pfc=# \d prova
            Table "public.prova"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 codi   | integer               | not null
 nom    | character varying(30) |
Indexes: prova_pkey primary key btree (codi)

pfc=# explain select * from prova where codi=1234;
                               QUERY PLAN
-------------------------------------------------------------------------
 Index Scan using prova_pkey on prova
(cost=0.00..5.99 rows=1 width=37)
   Index Cond: (codi = 1234)
(2 rows)

Now the query uses index, explain tell something about
index scan using index prova_pkey.

What is the diference with two cases? What must I do?
It is a bug? I need do something else?

Thanks a lot for helping me.

Regards,

Xevi.

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