def ReadPathInfosHistories(self,
                             client_id,
                             path_type,
                             components_list,
                             cursor=None):
    """Reads a collection of hash and stat entries for given paths."""
    # MySQL does not handle well empty `IN` clauses so we guard against that.
    if not components_list:
      return {}

    path_infos = {components: [] for components in components_list}

    path_id_components = {}
    for components in components_list:
      path_id = rdf_objects.PathID.FromComponents(components)
      path_id_components[path_id] = components

    # MySQL does not support full outer joins, so we emulate them with a union.
    query = """
    SELECT s.path_id, s.stat_entry, UNIX_TIMESTAMP(s.timestamp),
           h.path_id, h.hash_entry, UNIX_TIMESTAMP(h.timestamp)
      FROM client_path_stat_entries AS s
 LEFT JOIN client_path_hash_entries AS h
        ON s.client_id = h.client_id
       AND s.path_type = h.path_type
       AND s.path_id = h.path_id
       AND s.timestamp = h.timestamp
     WHERE s.client_id = %(client_id)s
       AND s.path_type = %(path_type)s
       AND s.path_id IN %(path_ids)s
     UNION
    SELECT s.path_id, s.stat_entry, UNIX_TIMESTAMP(s.timestamp),
           h.path_id, h.hash_entry, UNIX_TIMESTAMP(h.timestamp)
      FROM client_path_hash_entries AS h
 LEFT JOIN client_path_stat_entries AS s
        ON h.client_id = s.client_id
       AND h.path_type = s.path_type
       AND h.path_id = s.path_id
       AND h.timestamp = s.timestamp
     WHERE h.client_id = %(client_id)s
       AND h.path_type = %(path_type)s
       AND h.path_id IN %(path_ids)s
    """

    params = {
        "client_id": db_utils.ClientIDToInt(client_id),
        "path_type": int(path_type),
        "path_ids": [path_id.AsBytes() for path_id in path_id_components]
    }

    cursor.execute(query, params)
    for row in cursor.fetchall():
      # pyformat: disable
      (stat_entry_path_id_bytes, stat_entry_bytes, stat_entry_timestamp,
       hash_entry_path_id_bytes, hash_entry_bytes, hash_entry_timestamp) = row
      # pyformat: enable

      path_id_bytes = stat_entry_path_id_bytes or hash_entry_path_id_bytes
      path_id = rdf_objects.PathID.FromBytes(path_id_bytes)
      components = path_id_components[path_id]

      timestamp = stat_entry_timestamp or hash_entry_timestamp

      if stat_entry_bytes is not None:
        stat_entry = rdf_client_fs.StatEntry.FromSerializedString(
            stat_entry_bytes)
      else:
        stat_entry = None

      if hash_entry_bytes is not None:
        hash_entry = rdf_crypto.Hash.FromSerializedString(hash_entry_bytes)
      else:
        hash_entry = None

      path_info = rdf_objects.PathInfo(
          path_type=path_type,
          components=components,
          stat_entry=stat_entry,
          hash_entry=hash_entry,
          timestamp=mysql_utils.TimestampToRDFDatetime(timestamp))

      path_infos[components].append(path_info)

    for components in components_list:
      path_infos[components].sort(key=lambda path_info: path_info.timestamp)

    return path_infos