Is full-row updates slower than single-value updates

Hello everyone,

My question is like the subject, is it much slower to update all
columns values than just a single column? Generated update queries
from ORM:s generally have the following format:

update foo set a = 1, b = 2, c = 3, .... where id = 1234;

So each column is touched which, I suspect, is much slower than just
updating a single column because more indexes has to be rebuilt and so
on. EXPLAIN ANALYZE does not give any hints and claims that both
queries are equally fast. Maybe someone can guide me to documentation
which explains how to estimate how costly row-level updates are?

--
mvh Björn