libpq questions

For the libpq interface:

I need to be able to know if a column in a result from a query is
nullable or not. From reading the documentation it seems that I can
obtain the following information:
. scan all the rows in the result and see if there exists a null value
for each column...
. backtrack the column to the source table (assuming a non-calculated
field) and check the nullable status there

Neither of the above is particularly cheap to do...

Which leads me to my next question... If I executed a select against a
table with a million rows, and the query returned all of the rows, what
happens? Are all the rows read into memory on the client before
returning the result? Or are rows only fetched from the server as
required?

Thanks

James