Mass updates on a large table

Good afternoon,

I am attempting an update on two new date field columns on a 17 million row table.  Every row gets updated.

The update statement is a simple one:

  UPDATE EMAILRCPTS SET ID = ID

 And the update of the new date fields themselves occurs as the result of a before trigger.

The update took 3 days, 10 hours to complete on the testing box.  I have already adjusted the CHECKPOINT_SEGMENTS parameter up by 3X

To minimize the impact of checkpoints.   The SHARED_BUFFERS parameter has been bumped up to 140000 on a 20meg RAM box.

There are about 9 indexes on this table although none of them reference the date fields so since there are no inserts I don’t think they would have an impact on the update  (I’ve been wrong before though)

Would an update statement referencing the date fields work faster than a trigger?  Do you have any other suggestions to speed this up?

We simply cannot afford this table to be down for 3+ days during a production update.  The production box is a 32meg RAM box.

We are at Postgres 7.4.5.

Thanks for your time and brainpower