1. An apparatus comprising: one or more processors; and a computer readable storage medium having computer readable program code embodied therewith and executable by the one or more processors, the computer readable program code comprising: computer readable program code configured to receive a partial user query phrase, the partial query phrase comprising a final word element; computer readable program code configured to access a vocabulary; computer readable program code configured to ascertain from the vocabulary, at least one possible completion of the final word element; computer readable program code configured to access a phrase index derived from a document corpus; computer readable program code configured to ascertain, from the phrase index, at least two phrases corresponding to completions of the partial user query phrase with the at least one possible completion of the final word element; computer readable program code configured to assign a probability score to each of the at least two phrases; and computer readable program code configured to rank the at least two phrases based on probability; wherein to assign a probability score comprises determining a phrase selection probability, the phrase selection probability corresponding to the probability that a given phrase from the phrase index represents a completion of the partial query phrase given the final word element; wherein to assign a probability score comprises determining a phrase-query correlation, the phrase-query correlation corresponding to a measurement of correlation between a given phrase from the phrase index and that portion of the partial query phrase preceding the final word element; and wherein the phrase-query correlation comprises a function of a probability of observing a phrase from the phrase index alone and of a probability of a joint occurrence between a phrase from the phrase index and the portion of the partial query phrase preceding the final word element.