Column aliases for GROUP BY and HAVING

(Postgres version 8.2.4)

Trying to understand GROUP BY, I'm reading on
<http://www.postgresql.org/docs/8.4/interactive/queries-table-expressions.html>.
Consider this query:

=> select x, sum(y) from test1 group by x order by x;
x | sum
---+-----
a | 4
b | 5
c | 2
(3 rows)

I understand what's happening here, all is fine. So I play with HAVING:

=> select x, sum(y) from test1 group by x having sum(y)>3 order by x;
x | sum
---+-----
a | 4
b | 5
(2 rows)

But this confuses me:

=> select x, sum(y) as foo from test1 group by x having foo>3 order by x;
ERROR: column "foo" does not exist
LINE 1: ...ect x, sum(y) as foo from test1 group by x having foo>3 orde...

Why isn't it possible to refer to a column alias in HAVING?

--
- Rikard