[semiquaver]

Postgres' MVCC architecture makes the idea of 'partial updates' kind of meaningless. An update to any postgres column is equivalent to a DELETE followed by an INSERT and will result in a new row on disk, leaving the old row to be vacuumed later.

[morgo]

Just to clarify: s/architecture/implementation/

InnoDB is MVCC too, but does an update in place with relocation of the old row to UNDO space. There are pros and cons to both approaches.