Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for storing a plurality of documents in computer-readable memory, each document of the plurality of documents having a corresponding access control list (ACL), each ACL defining a plurality of users that are authorized to access a respective document, generating an index based on the plurality of users, the index comprising a plurality of partitions, each partition corresponding to a user of the plurality of users, and, for each document of the plurality of documents: ranking the users of the plurality of users, selecting a user as an indexing user based on the ranking, and storing the document in a partition of the index, the partition corresponding to the indexing user.