Databases have metadata to tell the database where to find the
data. Hadoop is the same. For the most part, its metadata is stored
in HCatalog. And all the "SQL on Hadoop" solutions use it to locate
data, determine how it's laid out so it can be accessed in
parallel, and what the schema is. But if you want to join data in
Hadoop with data in your RDBMS, then you need a unified view of
that metadata. If you're just "SQL on Hadoop" then you don't have
that view, and you need...