  Ontological query answering is the problem of answering queries in the
presence of schema constraints representing the domain of interest. Datalog+/-
is a common family of languages for schema constraints, including
tuple-generating dependencies (TGDs) and equality-generating dependencies
(EGDs). The interplay of TGDs and EGDs leads to undecidability or
intractability of query answering when adding EGDs to tractable Datalog+/-
fragments, like Warded Datalog+/-, for which, in the sole presence of TGDs,
query answering is PTIME in data complexity. There have been attempts to limit
the interaction of TGDs and EGDs and guarantee tractability, in particular with
the introduction of separable EGDs, to make EGDs irrelevant for query answering
as long as the set of constraints is satisfied. While being tractable,
separable EGDs have limited expressive power.
  We propose a more general class of EGDs, which we call "harmless", that
subsume separable EGDs and allow to model a much broader class of problems.
Unlike separable EGDs, harmless EGDs, besides enforcing ground equality
constraints, specialize the query answer by grounding or renaming the labelled
nulls introduced by existential quantification in the TGDs. Harmless EGDs
capture the cases when the answer obtained in the presence of EGDs is less
general than the one obtained with TGDs only. We conclude that the theoretical
problem of deciding whether a set of constraints contains harmless EGDs is
undecidable. We contribute a sufficient syntactic condition characterizing
harmless EGDs, broad and useful in practice. We focus on Warded Datalog+/- with
harmless EGDs and argue that, in such fragment, query answering is decidable
and PTIME in data complexity. We study chase-based techniques for query
answering in Warded Datalog+/- with harmless EGDs, conducive to an efficient
algorithm to be implemented in state-of-the-art reasoners.
