Any search phrase consisting of a single term only can be handled by
the PostingDB alone, and a document with single terms can never match
a real phrase (consisting a multiple terms).
Storing these terms also in the PositonDB just wastes space, so remove
these from the DB, and delegate single term phrases to the PostingDB.
While the document content and even filenames (basename + suffix) consist
of multiple terms typically, folder names, tags and properties are often
a single term.