Is there any method to limit resource usage in PG?

Hello:

Sorry for disturbing.

I am now encountering a serious problem： memory is not enough.

My customer reported that when they run a program they found the totall
memory and disk i/o usage all reached to threshold value(80%).

That program is written by Java.
It is to use JDBC to pull out data from DB, while the query joined some
table together, It will return about 3000,000 records.
Then the program will use JDBC again to write the records row by row , to
inert into another table in the DB.

My first question is:

Currently, my customer can not update there applications , and also they
have no budget to buy new hardware.

In fact, my customer want the program 's JDBC related part can be dealed
with by PG at this way:
The PG can control all of its process to run under usage threshold.
If the resource will reach the threshold, then PG will slow processes
down under the threshold.

I know that in Oracle, that there are resource plan etc. to approach this.
But in PG, I haven't find it.

And I also want to know:

When there are so many records need to be thrown to client,
will PG use as much memory as it can?
If the total amount of records is larger than shared_buffer, will PG hung?
or just do its work and communicate with client for a few times till all
the data is transferred?

Best Regards