Updates: all or partial records

Scenario: You have to update a record. One or more fields are unchanged
from the original record being altered. So you have two options: 1)
Include those fields in your UPDATE statement, even though they are
unchanged; 2) Omit unchanged fields from the UPDATE statement.

My first inclination is to omit unchanged fields. However, I have the
idea that PG simply marks the existing record to be dropped, and
generates a whole new row by copying unspecified fields from the
original record.

My question is, which is more efficient? Performance-wise, does it
matter whether unchanged fields are included or omitted on UPDATE
statements?

Paul

-- 
Paul M. Foster