According to the article, CPLEX has a specific solver for mixed quadratic optimization problems. Maybe this answer my question. Thank you, Mrs Parsapoor.
Integer programming with quadratic constraints is "worse" than NP-hard. In 1973, Robert G. Jeroslow proved that it is "undecidable", which means that there cannot exist any algorithm that is guaranteed to terminate in finite time.
Nevertheless, in practice, most problems have bounded variables. In that case, the problem becomes "merely" NP-hard.
You might find this survey paper useful:
S. Burer & A.N. Letchford (2012) Non-convex mixed-integer nonlinear programming: a survey. Surveys in Oper. Res. and Mgmt. Sci., 17(2), 97-106.
CPLEX V12.6.0 is capable of solving a quadratic program (that is, a problem with linear constraints and one or more quadratic terms in the objective function) for which the space of optimal solutions is not convex to global optimality. Such problems are known as nonconvex QPs. In addition to the linear constraints and quadratic objective terms, the problem can also include integer constraints, and CPLEX V12.6.0 can solve the MIQP to global optimality.