cursor empty

Hello,

I have got a function with this definition:

CREATE OR REPLACE FUNCTION vectorize(refcursor)
RETURNS SETOF refcursor
LANGUAGE 'plpgsql'
COST 100
STABLE
ROWS 1000
AS $BODY$

begin
perform pivottable(
'_vector',
'select * from viewdata',
array['ideate', 'name', 'description', 'latitude', 'longitude'],
array['parametername'],
array['parametervalue::text', 'parametervaluetext']
);
open $1 scroll for select * from _vector;
return next $1;
end

$BODY$;

The perform call creates a dynamic column pivot table, if I run manually

select pivottable(
'_vector',
'select * from viewdata',
array['ideate', 'name', 'description', 'latitude', 'longitude'],
array['parametername'],
array['parametervalue::text', 'parametervaluetext']
);
select * from _vector;

I get all the data in the output, so everything is fine.

My goal is now to encapsulate the two lines into a function, so I define
a stable function and based on the dynamic column set a cursor. I get in
pgadmin the column names back, but the rows are empty if I run:

select * from vectorize('myvec');
fetch all from myvec;

Can you explain me, which part is wrong?
Thanks

Phil