what if PQexec() result is toooooo big?

What happens when the PQexec(..) result is too big to return in one single
response message? Say you are reading a table with millions of entries. Is
there a way to write one single PQexec() request but read the result in
chunks :

  "give me the first 500 tuples",
  "give me the next 500",
   and so on... until you read the entire table.

How does postgres handle these situations?

Ajay