  We answer the question which conjunctive queries are uniquely characterized
by polynomially many positive and negative examples, and how to construct such
examples efficiently. As a consequence, we obtain a new efficient exact
learning algorithm for a class of conjunctive queries. At the core of our
contributions lie two new polynomial-time algorithms for constructing frontiers
in the homomorphism lattice of finite structures. We also discuss implications
for the unique characterizability and learnability of schema mappings and of
description logic concepts.
