PGresult pointer and memory usage

When you do a PQexec call in libpq, does PostgreSQL produce the entire
result in memory on the server side, and then does libpq hold the entire
result in memory after it retrieves it?

The only time that the server sends parts of the result at a time is
with a cursor, right? And libpq always holds the data that it gets back,
right?

I guess what I'm asking is, where is the PGresult data actually stored,
and how much of it is stored in the client application at a time?

Regards,
    Jeff Davis