Declare row type variable in PL/pgSQL

As I found SELECT * FROM t INTO my_data; works only if:

DO $$
DECLARE
my_data t%ROWTYPE;
BEGIN
SELECT * FROM t INTO my_data WHERE id = ?;
END $$;

Am I right?

If I want to get only 2-3 columns instead of all columns. How can I define my_data?

That is,

DO $$
DECLARE
my_data <WHAT HERE??>;
BEGIN
SELECT id,name,surname FROM t INTO my_data WHERE id = ?;
END $$;

get only 2-3 columns instead of all columns

One way: use a record variable:

DO
$do$
DECLARE
   _rec record;
BEGIN
   SELECT INTO _rec
          id, name, surname
   FROM   t
   WHERE  id = ?;

   -- do something with it ...
END
$do$;

Note that the structure of a record type is undefined until assigned. So you cannot reference columns (fields) before you do that.

Another way: assign multiple scalar variables:

DO
$do$
DECLARE
   _id int;
   _name text;
   _surname text;
BEGIN
   SELECT INTO _id, _name, _surname
          id,  name,  surname
   FROM   t
   WHERE  id = ?;
END
$do$;

%ROWTYPE is just noise in Postgres. The manual:

(Since every table has an associated composite type of the same name, it actually does not matter in PostgreSQL whether you write %ROWTYPE or not. But the form with %ROWTYPE is more portable.)

So:

DO
$do$
DECLARE
   my_data t;  -- table name serves as type name, too. 
BEGIN
   SELECT INTO my_data * FROM t WHERE id = ?;
END
$do$;