Adding fields on the fly

I am investigating the possibility of using pgsql as the database in an application.  I have some unusual requirements
thatI'd like to ask you all about.  I apologize in advance if my terminology is a little "off", I'm not familiar with
pgsql(yet). 

My first requirement is that I need to be able to add new columns to any database table on the fly.  By this I mean
thatI need this to happen interactively -- a user will drag some UI widget from here to there and boom! my application
willrequest a new column to be added to a table, and wait for it to happen before giving visual feedback to the user.
Myquestions are: 

Is this even possible? Is ALTER TABLE supported?
If so, what performance characteristics can I expect?  Compared to, say, inserting a record, can I expect this to be 10
timesslower? 1000 times slower? 
What does the performance depend on?  Number of rows? Number of columns? Number of tables? anything else?


I'll send my other question in a separate message to keep the threads separate.

Thanks in advance,

Gail Zacharias