Stable function Repeatedly Called

Hi!

drop table tt1;
create table tt1 (c1 text,c2 text);
drop table tt2;
create table tt2 (c1 text,c2 text);
insert into tt1 values('a','aa');
insert into tt1 values('b','bb');
insert into tt1 values('c','cc');
insert into tt2 values('x','a');
insert into tt2 values('y','b');

CREATE OR REPLACE FUNCTION f1(TEXT) RETURNS TEXT AS'
DECLARE
  MyNumber TEXT;
BEGIN
  SELECT c2 INTO MyNumber FROM tt2 WHERE c1 = $1;
  RAISE NOTICE ''%'',MyNumber; --for debug
  RETURN MyNumber;
END' LANGUAGE 'plpgsql' STABLE;

select * from tt1 where c1=f1('x');
update tt1 set c2='ABC' WHERE c1=f1('x');

I thought function "f1" would get called only once but actually it is called 3 times. How do I force it to be called
onlyonce in the same SQL? 
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