(with Postgis 1.4)I have a simple update query that takes hours to run.

For small numbers of rows requiring updates, it can be adequate to use an UPDATE statement for each row that requires an update.

That gives about 1GB of data, so with a bit of overhead I'd say about 1.5GB.

Fabrice, have you done some monitoring (iostat, dstat, ...) when the update was in progress?

The decreased result was similar to earlier Vertica releases before optimized MERGE was available.

Here are the benchmark results when we met all the optimized query plan conditions.