Support for using alias in having clause

[Michael Lewis <mlewis@xxxxxxxxxxx>]

Why can I use an alias for group by and order by and not for having? I am just wondering if there is some barrier to it being implemented, or if it just hasn't been.

select
table_schema || '.' || table_name AS schema_qualified_table_name,
count( column_name ) as column_count
from
information_schema.columns
group by
schema_qualified_table_name
having
count( column_name ) > 50 /* this works, but trying column_count > 50 does not */
order by
column_count desc;