1. A method of indexing documents, the method comprising: extracting, by a computer processor, feature information from each of a plurality of documents to be indexed; defining a plurality of indexes based on statistical properties of the extracted feature information, wherein the defining includes establishing, for each of the indexes, an upper limit on each of one or more parameters measuring a size of the index; for each of at least some of the documents: selecting one of the indexes as a destination index for the document based on the feature information extracted from the document, wherein the selecting includes: identifying a current index from among the plurality of indexes; determining, for each of the one or more parameters, whether adding the document to the current index will result in the index exceeding the upper limit on the parameter; and selecting the current index as the destination index for the document in the event that adding the document to the current index will not result in the index exceeding the upper limit on any of the one or more parameters; and adding a searchable representation of the document to the destination index; and storing the plurality of indexes in a computer-readable storage medium.