I think the parent commenter might be referring to systems like Hive or HBase built on top of Hadoop and do have a lot of overlap with a large scale database system.
Exactly, thanks. "MapReduce paradigm" wasn't precise, and I can see why that makes everyone want to give a distributed systems 101 lecture in this comment thread :)
MapReduce does not have a set in stone data source/sink and can use multiple things like bigtable and spanner so they are complementary technologies.