field alias in where condition

Hi all,

I issued the following queries:

select substring(proname from 1 to 1) as nevresz, count(*)
from pg_proc
where nevresz = 'a'
order by nevresz
group by nevresz;

select substring(proname from 1 to 1) as nevresz, count(*)
from pg_proc
order by nevresz
group by nevresz;

The first query fails, and says that column 'nevresz' does not exist.
The second is OK.

What is the problem? I cannot use column alias in where condition?

Thanks,
Otto