query large amount of data in c++ using libpq

Hi all,
i am using PQexecParams() to "SELECT" about 3 million record in C++, and it
takes several minutes to make it done with used memory dramatically
incresed(about 200MB).
it seems when i using PQexecParams(), i can't use the query result before
all the data is sent to client.
is there something like server side cursor in libpq?
or is there any other way to do this in c++?
some articles say that query for large amount of data is automaticall
handled well by pg, but i can't see how.

thanks~~~~