Large Queryset Calculation In Background?

Hi folks --

I need to run a fairly CPU-intensive calculation nightly over a
dataset that's already large and growing quickly. I'm planning to run
this via a cron job, but would like to make sure that it neither eats
up the entire CPU nor locks the database, so that my site can continue
functioning in the meantime. The rough outline of what it needs to do
is as follows:

class OtherThing(models.Model):
anotherthing = models.ManyToManyField(Whatever)
...

class Thing(models.Model):
other_things = models.ManyToManyField(OtherThing,
through='SomethingElse')
...

for thing in Thing.objects.select_related('other_things',
'other_things__anotherthing__etc'):
calculated = calculation_on_thing_and_its_otherthings(thing) #
this mainly involves serialization to a great depth
thing.calculated_data = calculated
thing.save()

Will the above approach lock the database for a while or eat tons of
CPU? Any suggestions? I'm using Django 1.2, btw.

Thanks,
-Nan

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