some hints to understand the plsql cursor.

actually I'm hacking pg for a function like :
1. define a select query.
2. client ask for some data. and server reply some data. server will do
NOTHING if client doesn't ask any more..
3. client ask some data more data with a batch and SERVER reply some data
then. then do NOTHING.

currently the simple "select * from t", the server will try to send the
data to client at one time which is not something I want.

by looking into the plsql, looks it has some api like:

fetch 10 from cursor_1;
fetch 10 from cursor_1;

I'm lacking of the experience to hack plsql. so my question are:
1. Does pg has some codes which act like the "ask -> reply -> ask again
-> reply again" on the server code? currently I'm not sure if the above
"fetch" really work like this.
2. any resources or hint or suggestion to understand the "fetch"
statement?

Thanks