Best approach to run a simple query against all users

One way to do this would be Object Types, in this case the type would be analagous to your #t1. So it would need to be defined somewhere but it would not need to be global, it could be per-schema or per-procedure even. First, we can create a type:

SQL> create or replace type t1_type as object (x int, y int, z int)
  2  /

Type created.

SQL> create or replace type t1 as table of t1_type
  2  /

Type created.

Now set up some sample data:

SQL> create table xy (x int, y int)
  2  /

Table created.

SQL> insert into xy values (1, 2)
  2  /

1 row created.

SQL> insert into xy values (3, 4)
  2  /

1 row created.

SQL> commit
  2  /

Commit complete.

And create a function over this data returning our "temporary" type:

SQL> create or replace function fn_t1 return t1 as
  2  v_t1 t1 := t1();       -- empty temporary table (really an array)
  3  v_ix number default 0; -- array index
  4  begin
  5  for r in (select * from xy) loop
  6  v_ix := v_ix + 1;
  7  v_t1.extend;
  8  v_t1(v_ix) := t1_type(r.x, r.y, (r.x + r.y));
  9  end loop;
 10  return v_t1;
 11  end;
 12  /

Function created.

And finally:

SQL> select * from the (select cast (fn_t1 as t1) from dual)
  2  /

         X          Y          Z
---------- ---------- ----------
         1          2          3
         3          4          7

As you can see this is pretty clunky (and uses collection pseudo-functions, which is an obscure feature at the best of times!), as I always say, porting from DB to DB is not merely about syntax and keywords in their SQL dialects, the real difficulty comes in different underlying assumptions (in the case of SQL Server, that cursors are expensive and their use avoided/worked around at all costs).

It seems like you want something like

select *
  from (select person_id,
               rating_date,
               rating,
               lag(rating) over (partition by person_id
                                     order by rating_date) prior_rating
         from rating)
 where rating != prior_rating
    or prior_rating is null
 order by person_id, rating_date

which yields the following

SQL> ed
Wrote file afiedt.buf

  1  select person_id,
  2         rating_date,
  3         rating
  4    from (select person_id,
  5                 rating_date,
  6                 rating,
  7                 lag(rating) over (partition by person_id
  8                                       order by rating_date) prior_rating
  9           from rating)
 10   where rating != prior_rating
 11      or prior_rating is null
 12*  order by person_id, rating_date
SQL> /

 PERSON_ID RATING_DAT RATING
---------- ---------- --------
         1 2010-08-04 A3
         1 2010-08-14 A1
         2 2010-08-04 G2
         2 2010-08-14 G1
         2 2010-08-20 G4
         3 2010-08-04 G1
         3 2010-08-14 G3

7 rows selected.