A method includes obtaining data from a plurality of data sources associated with an n-gram indexing data structure and storing at least a portion of the obtained data in a first storage, the stored data comprising one or more n-gram strings. The method also includes estimating frequencies of occurrence of respective ones of the n-gram strings in the stored data, the estimated frequency of occurrence of a given n-gram string being based at least in part on a size of a given n-gram index in the n-gram indexing data structure corresponding to the given n-gram string. The method further includes, in response to detecting one or more designated conditions, selecting a portion of the stored data based at least in part on the estimated frequencies and moving the selected portion of the stored data from the first storage to a second storage having different read and write access times.