The thought:
- Load the big chunk of data into a new table
- Generate some minimal set of indices on the new table
- Generate four queries that compare old to new:
q1 - See which tuples are unchanged from yesterday to today
q2 - See which tuples have been deleted from yesterday to today
q3 - See which tuples have been added
q4 - See which tuples have been modified
If the "unchanged" set is extremely large, then you might see benefit
to doing updates based on deleting the rows indicated by q2,
inserting rows based on q3, and updating based on q4.
In principle, computing and applying those 4 queries might be quicker
than rebuilding from scratch.
In principle, applying q2, then q4, then vacuuming, then q3, ought to
be "optimal."