Two-particle momentum correlations of N identical bosons are studied in the quantum canonical ensemble. We define the latter as properly selected sub-ensemble of events associated with the grand canonical ensemble which is characterized by a constant temperature and a harmonic-trap chemical potential. The merits of this toy-model are that it can be solved exactly, and that it demonstrates some interesting features revealed recently in small systems created in p + p collisions at the LHC.We find that partial coherence can be observed in particles emission from completely thermal ensemble of events if instead of inclusive measurements one studies the two-boson distribution functions related to the events with particle numbers selected in some fixed multiplicity bins. The corresponding coherence effects increase with the multiplicity.