Compute group proportions of a whole in SQL

It's easy with SQL to use aggregates to count members of a group. But what if you also want the proportion that group is of the whole. The answer is to use a window function combined with the aggregate. The window function is evaluated after the normal aggregation. i.e. it can reference the values of other post-aggregation rows.

select type, count(*), count(*)::numeric / sum(count(*)) over ()
from pets
group by type;

  type  | count |        ?column?        
--------+-------+------------------------
 dog    |    13 | 0.52000000000000000000
 rabbit |     3 | 0.12000000000000000000
 cat    |     9 | 0.36000000000000000000