data type

Hi,
I am writing some function in postgres pl/sql.

My function is of type St_ABC((select obj_geom from XYZ),(select
boundary_geom from boundary))
I have table XYZ with 20,000 tuples and in boundary, i have only one
geometry.

In postgres, ST_intersects(obj_geom, boundary_geom) checks each obj_geom
with boundary_geom and returns true/false. It returns true/false 20,000
times
I want to write function that return only one true/false according to my
calculation.

So, create or replace function ST_ABC(?????, geometry) returns boolean

Which type of data type will be used in above function (in place of ?????)
that can collect more than one row(20,000) ?

Thanks
Raj