> I want to use an aggregate function that will return the most commonly
> occurring value in a column.
It's actually dead simple in Postgres. No C either. You just need to create an aggregate function.
I wrote a most() aggregate a while back that does exactly what you are asking for. Here, I'll add it to my blog...
http://scottrbailey.wordpress.com/2009/05/22/postgres-adding-custom-aggregates-most/
Scott Bailey