The UNION caused the query to be run in a way that is very
inefficient on MySQL, separating the queries causes each of them to
run in a much more efficient form.
Any duplicated will be filtered out on the PHP side instead of in
the database. On large Moodle sites this is preferable as the
extract is likely to be performed on a server dedicated to running
the Moodle cron and so there will be less of effect on resources
that are used to serve end users.
Neill Magill