> First off, what does it say for rows affected? (Hint, if you really
> are using a default configuration and it doesn't say 0 rows
> affected, please show us the actual query used.)
update imports set make_id = null
Query returned successfully: 98834 rows affected, 49673 ms execution time.
vacuum analyze imports
Query returned successfully with no result in 4138 ms.
VACUUM FULL imports;
Query returned successfully with no result in 38106 ms.
VACUUM FREEZE ANALYZE;
Query returned successfully with no result in 184635 ms
update imports set make_id = 0
Query returned successfully: 98834 rows affected, 45860 ms execution time.
So all the vacuuming saved about four seconds of execution time.
here is the postgresql.conf completely untouched from the default
install https://gist.github.com/4590590