  We consider the evaluation of first-order queries over classes of databases
with bounded expansion. The notion of bounded expansion is fairly broad and
generalizes bounded degree, bounded treewidth and exclusion of at least one
minor. It was known that over a class of databases with bounded expansion,
first-order sentences could be evaluated in time linear in the size of the
database. We give a different proof of this result. Moreover, we show that
answers to first-order queries can be enumerated with constant delay after a
linear time preprocessing. We also show that counting the number of answers to
a query can be done in time linear in the size of the database.
