a heavy duty operation on an "unused" table kills my server

Hi list, I'm having a problem when dealing with operations that asks too much CPU from the server.
The scenario is this:

I have a multithreaded server, each thread with its own connection to the database. Everything is working fine, actually great, actually outstandingly, in normal operation.

I've a table named "a" with 1.8 million records, and growing, but I'm ok with it, at least for the moment. Maybe in the near future we will cut it down, backup old data, and free it up. But this is not the issue, as I said, everything is working great. I have a cpl of indexes to help some queries, and that's it.

Now my problem started when I tried to do some model refactoring on this production table.

First I tried a dumb approach.
I connected from pgadmin, opened a new session.
I tried an ALTER TABLE on this table just to turn a char(255) field into char(250), and it locked up my system.

No surprise, since I had many threads waiting for this alter table to finish. What I did not foresee was that this alter table would take up so much time. Ok, my fault, for not having calculated the time that it would take the ALTER TABLE to complete.

Now, with this experience, I tried a simple workaround.
Created an empty version of "a" named "a_empty", identical in every sense.
renamed "a" to "a_full", and "a_empty" to "a". This procedure costed me like 0 seconds of downtime, and everything kept working smoothly. Maybe a cpl of operations could have failed if they tried to write in the very second that there was actually no table named "a", but since the operation was transactional, the worst scenario was that if the operation should have failed, the client application would just inform of the error and ask the user for a retry. No big deal.

Now, this table, that is totally unattached to the system in every way (no one references this table, its like a dumpster for old records), is not begin accessed by no other thread in the system, so an ALTER table on it, to turn a char(255) to char(250), should have no effect on the system.

So, with this in mind, I tried the ALTER TABLE this time on the "a_full" (totally unrelated) table.
The system went non-responsive again, and this time it had nothing to do with threads waiting for the alter table to complete. The pgAdmin GUI went non-responsive, as well as the application's server GUI, whose threads kept working on the background, but starting to take more and more time for every clients request (up to 25 seconds, which are just ridiculous and completely unacceptable in normal conditions).

This resulted in my client applications to start disconnecting after their operations failed due to timeout, and the system basically went down again, from a users point of view.

This time, since I saw no relation between my operation on a totally unrelated table, and the server BIG slowdown, I blamed the servers memory.

After some tests, I came up to the conclusion that any heavy duty operation on any thread (ALTER TABLE on 1.8 million records tables, updates on this table, or an infinite loop, just to make my point), would affect the whole server.

Bottom line is, I can't seem to do any heavy processing on the database (or any operation that would require the server to enter into high CPU usage), and still expect the server to behave normally. Whatever heavy duty operation, DDL, DML, on whatever table (related, or unrelated), on whatever thread, would tear down my servers integrity.

My question then is: is there a way to limit the CPU assigned to a specific connection?
I mean, I don't care if my ALTER TABLE takes 4 days instead of 4 hours.

Something like:
pg_set_max_cpu_usage(2/100);

and rest assured that no matter what that thread is asking the database to do, it just wont affect the other running threads. Obviosly, assuring that the process itself does not involve any locking of the other threads.

Is something like that possible?

Thanks in advance,
Eduardo.