To time the execution of various stages of a long transaction, I’m using the following function:

CREATE OR REPLACE FUNCTION CVL_TimerLap() RETURNS double precision AS $$
import time
now = time.time()
if not SD.has_key('t_last'):
  SD['t_last'] = now
elapsed = now - SD['t_last']
SD['t_last'] = now
return elapsed
$$ LANGUAGE plpythonu;

The “lap times” are returned using:

SELECT CVL_TimerLap();

This will return the number of seconds (wall-clock time) that have passed since the function was last invoked.