18. The non-transitory computer-readable storage medium of claim 15 , wherein counts of candidate phrases are weighted with respect to age.
15. A non-transitory computer-readable storage medium storing instructions that, when executed by a processor, cause the processor to: associate keywords of previous searches with a topic; identify a candidate phrase from the previous searches that contains a first keyword and at least one second keyword associated with the topic; determine a first count of the first keyword, a second count of the at least one second keyword, and a mutual count of the first keyword and the second keyword appearing simultaneously in the previous searches; weight each individual count of the first count, the second count, and the mutual count based at least in part on age of a respective individual count with respect to the previous searches, wherein each age is a difference between a time when the respective individual count appeared in the previous searches and a current time; determine pointwise mutual information score for the candidate phrase using the weighted first count of the first keyword, the weighted second count of the second keyword, and the weighted mutual count of the first keyword and the second keyword; based at least in part on the determined pointwise mutual score being greater than a threshold pointwise mutual score, select the candidate phrase as a fixed phrase; determine a relevance score for the fixed phrase with respect to the topic; based at least in part on the relevance score being above a threshold score for the topic, associate the fixed phrase with the topic; in response to receiving a search query, provide at least one search result for presentation, the at least one search result at least referencing content selected from a search index based at least in part on relevance scores of a plurality of keywords and a plurality of fixed phrases for the topic; detect interaction with a search result of the provided at least one search result from a user; and update the keywords associated with the search result.