How to get the requested columns in a rowtype PL/pgSQL function

Hi all,

Imagine I have a PL/pgSQL function called myfunction that returns a
rowtype like this

CREATE TYPE mytype AS (a integer, b integer, c integer);

Imagine that I often need only 1 or 2 of this values, just say I need to
do query like these :

select a from myfunction()
select a,c from myfunction()
select b,c from myfunction()

and so on.

Now, since computing a,b and c could be expensive in term of time, is
there a way to understand (from withing the PL/pgSQL function) which
fields where requested in the query and then avoid computing those who
are not needed ?

Thanks in advance for every hints !

Ciao,
Federico.