1. A method for indexing content items based on click-through features, the method comprising: generating a training set comprising one or more query-content item pairs, wherein a given query-content item pair has one or more click-through features associated therewith; labeling one or more query-content item pairs in the training set by assigning click score thereto based on the one or more click-through features thereof; determining a click score function based on the click scores of the labeled query-content item pairs and the click-through features thereof; applying the click score function to a plurality of unlabeled query-content item pairs to determine click scores thereof based on the one or more click-through features of the unlabeled query-content item pairs.