Does Postgres rewrite entire row on update?

Question

We run Postgres 9.0 on Windows 2008 Server. There is a large table contains a bytea column for storing binary data ranging from 0-5MB in each row:

Answer 1

Picking c) from your questions:

Is there a way we can perform minor boolean field updates to this table WITHOUT rewriting the row (&chewing up diskspace) each time?

As @Craig already explained, columns that are "TOAST-able" and bigger than a certain threshold are stored out-of-line in a dedicated TOAST table per table (separate "relation forks", separate files on disk). So, a 5 MB bytea column would stay mostly untouched in an update if the column itself is not changed. The manual:

During an UPDATE operation, values of unchanged fields are normally preserved as-is; so an UPDATE of a row with out-of-line values incurs no TOAST costs if none of the out-of-line values change.

Bold emphasis mine.
The row in the main relation fork is still copied and a dead row stays behind when updated (whether or not any values actually changed). For large row sizes, the following solution might pay:

Create a small separate 1:1 table for frequently changed flags. Just the primary key (= foreign key at the same time) and the frequently changed flags. This would make updates a lot faster and preserve disk space - for an initial extra overhead and some cost for queries that need to join both tables (other queries actually get faster). More about on-disk space requirement of table rows:

• Making sense of Postgres row sizes