backend don't use index when querying by indexed column

Zdenek Habala (zhabala(at)telecom(dot)cz) reports a bug with a severity of 2
The lower the number the more severe it is.

Short Description
backend don't use index when querying by indexed column

Long Description
I have a table named formula with this structure:

--------------------
id_formula | integer | not null default nextval('"formula_id_formula_seq"'::text)
id_loan | integer |
cisfor | integer |
castpr | integer |
castuh | integer |
rt_datfor | integer |
rt_datspl | integer |
rt_datvlo | integer |
kdo | character(8) |
plod | timestamp with time zone |
pldo | timestamp with time zone |
Indices: formula_id_loan,
predpis_id_formula_key

Index "predpis_id_formula_key"
Attribute | Type
------------+---------
id_formula | integer
btree

when i try to explain this select i will got this:

explain select * from formula where id_formula = 1;
NOTICE: QUERY PLAN:
Seq Scan on formula (cost=0.00..10919.89 rows=4576 width=72)

but when i try to explain modified select like this:
explain select * from formula where id_formula = 1 and id_formula=id_formula;
NOTICE: QUERY PLAN:
Index Scan using predpis_id_formula_key on formula (cost=0.00..11952.57 rows=46
width=72)

backend uses the index.

Is it a bug or isn't it ? ( I have hundreds of other tables and any one of them have not this problem, only that formula one. )

Thanx for explanation.

Sample Code

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