def Query(cls, index_urn, target_prefix="", limit=100, token=None):
    """Search the index for matches starting with target_prefix.

    Args:
       index_urn: The index to use. Should be a urn that points to the sha256
         namespace.
       target_prefix: The prefix to match against the index.
       limit: Either a tuple of (start, limit) or a maximum number of results to
         return.
       token: A DB token.

    Returns:
      URNs of files which have the same data as this file - as read from the
      index.
    """
    return data_store.DB.FileHashIndexQuery(
        index_urn, target_prefix, limit=limit)