functional index to_char on the postgresql (to_char, immutable issue)

gistfile1.txt

CREATE OR REPLACE FUNCTION my_to_char(some_time timestamp) 
  RETURNS text
AS
$BODY$
    select to_char($1, 'yyyy-mm-dd');
$BODY$
LANGUAGE sql
IMMUTABLE;

CREATE INDEX fc_op_vist_idx_function_med_dt ON table (my_to_char(MED_DT, 'YYYY-MM'));