Postgresql COALESCE performance problem

I have this table in Postgresql:

CREATE TABLE my_table
(
    id bigint NOT NULL,
    value bigint,
    CONSTRAINT my_table_pkey PRIMARY KEY (id)
);

There are ~50000 rows in my_table.

The question is, why the query:

SELECT * FROM my_table WHERE id = COALESCE(null, id) and value = ?

is slower than this one:

SELECT * FROM my_table WHERE value = ?

Is there any solution, other than optimizing the query string in app-layer?

EDIT: Practically, the question is how to rewrite the query select * from my_table where id=coalesce(?, id) and value=? to have worst case performance not less than that of select * from my_table where value=? in Postgresql 9.0