>> Does Postgres allow updates based on the context of a sub-query,
something like the sample below ?
Yes,
Update real_tab
set real_tab.data_desc = temp_tab.data_desc
From temp_tab
Where real_tab.keyID = temp_tab.keyID
(don't repeat your updated table in the "from" list unless you
Mean to self-join)
http://www.postgresql.org/docs/8.3/interactive/sql-update.html