False unique constraint violation (exception block)

Hello.

This is simple example for describe my problem.

I created a table:

CREATE TABLE table0
(
id serial NOT NULL,
field0 integer,
field1 text
);

I created an unique index for this table:

CREATE UNIQUE INDEX idx_table0_unique
ON table0 ( field0, coalesce ( field1, 'INDEX_COLUMN_NULL' ) );

I created function for insert only unique record (part of code):

BEGIN
INSERT INTO table0 ( field0, field1 ) VALUES ( p_field0, p_field1 )
RETURNING id INTO v_table0_id;
EXCEPTION WHEN unique_violation THEN
SELECT id FROM table0 WHERE field0 = p_field0 AND field1 = p_field1;
END;

I use this function for add data to table and sometimes I getting false
unique violation (in block above). Why?

Thanks for replay.