A postgres conundrum

gistfile1.txt

#
# lets say we have a LARGE table 

test=# select count(*) from blah;
 count 
----------
 20000000
(1 row)

#
# and we want to add a column to it with a default value 

test=# alter table blah add column active boolean not null default false;
ALTER TABLE

#
# this can easily take hours (took about 4.5 hours)

#
# but ...

#
# this code takes no time 

test=# alter table blah add column active boolean;
ALTER TABLE

# and then this Ruby code takes 20 minutes (lets assume all ids are sequential)

20_000.times do |i|
  min = i*1000
  max = (i+1)*1000 - 1
  Blah.where( "id between (#{min}, #{max}").update_all('active = false')
end

# and then this SQL code takes seconds 
test=# alter table blah alter column active set default true;
ALTER TABLE
test=# alter table blah alter column active set default not null;
ALTER TABLE


#
# Yes, second examples looses transactionality ... but it is order of magnitude faster.

#
# Question: is there a Postgres way of doing it? 
#   Some sort of incremental commit? 
#   Some way of turning off transactionality for one update statement?
#
# Thanks for reading!