Monadic Datalog Containment on Trees
  We show that the query containment problem for monadic datalog on finite unranked labeled trees can be solved in 2-fold exponential time when (a) considering unordered trees using the axes child and descendant, and when (b) considering ordered trees using the axes firstchild, nextsibling, child, and descendant. When omitting the descendant-axis, we obtain that in both cases the problem is EXPTIME-complete. 
Andr\'e Frochaux and Martin Grohe and Nicole Schweikardt
cs.LO cs.CC cs.DB