  Higher security and lower failure probability have always been people's
pursuits in quantum-oblivious-key-transfer-based private query (QOKT-PQ)
protocols since Jacobi \emph{et al}. [Phys. Rev. A 83, 022301 (2011)] proposed
the first protocol of this kind. However, higher database security generally
has to be obtained at the cost of a higher failure probability, and vice versa.
Recently, based on a round-robin differential-phase-shift quantum key
distribution protocol, Liu \emph{et al}. [Sci. China-Phys. Mech. Astron.58,
100301 (2015)] presented a private query protocol (RRDPS-PQ protocol) utilizing
ideal single-photon signal which realizes both ideal database security and zero
failure probability. However, ideal single-photon source is not available
today, and for large database the required pulse train is too long to
implement. Here, we reexamine the security of RRDPS-PQ protocol under imperfect
source and present an improved protocol using a special "low-shift and
addition" (LSA) technique, which not only can be used to query from large
database but also retains the features of "ideal database security" and
"zero-failure" even under weak coherent source. Finally, we generalize the LSA
technique and establish a generic QOKT-PQ model in which both "ideal database
security" and "zero failure" are achieved via acceptable communications.
