On the Myria side, every subquery you actually execute is 1-1 with a Parallel. Parallel means "run these fragments in Parallel" -- the fragments are usually connected, but could be independent MyriaL statements. So we could put a Sequence there, but we'd still need a Parallel underneath it.

A hidden reason is that, since the profiling viz has not been updated to be subquery-aware, I only let you profile queries that are rooted in a single Parallel operator.