Keyword search against structured databases has become a popular topic of
investigation, since many users find structured queries too hard to express,
and enjoy the freedom of a ``Google-like'' query box into which search terms
can be entered. Attempts to address this problem face a fundamental dilemma.
Database querying is based on the logic of predicate evaluation, with a
precisely defined answer set for a given query. On the other hand, in an
information retrieval approach, ranked query results have long been accepted as
far superior to results based on boolean query evaluation.