1. A computer-implemented method facilitating a search of a collection of content, comprising: under control of one or more computer systems configured with executable instructions, associating keywords of previous searches of the collection of content with topics in accordance with a latent Dirichlet allocation; for each of the topics, identifying candidate phrases of the previous searches that contain at least one of the keywords associated with the topic; determining pointwise mutual information scores for the candidate phrases; and selecting at least some of the candidate phrases as fixed phrases based at least in part on a greatest pointwise mutual information score determined for the topic; determining relevance scores for each of at least some of the keywords and each of at least some of the fixed phrases with respect to the collection of content; and providing at least one result of the search for presentation, said at least one result at least referencing content selected from the collection of content based at least in part on the determined relevance scores.
5. A computer-implemented method according to claim 1 , wherein selecting the fixed phrases comprises: determining a pointwise mutual information score threshold based at least in part on the greatest pointwise mutual information score determined for the topic; and selecting candidate phrases with pointwise mutual information scores greater than the pointwise mutual information score threshold.