Yes, you can do it parallel (and in this circumstance it’s parallel query +
parallel DDL) and online. The main difference is that your reading of the
table can’t use direct path reads when it’s online.
If you can’t see parallel sessions being used then it’s likely because
there weren’t any free to be used. If you have diagnostic+tuning pack
licenced you can run the query on the bottom of
https://ctandrewsayer.wordpress.com/2017/10/19/how-many-rows-were-insertedupdateddeleted-in-my-merge/
against the sql_id to see the reported reason for DOP downgrade.
That said, direct path read for parallel query on a partitioned table won’t
necessarily happen anyway due to some bugs that have only recently been
addressed.
Hope that helps,
Andrew
