code: def _update_time_stamp(self, hash_value):
          
          db_name = self._database_from_key(hash_value)
          if not db_name:
              db_name=':memory:'
  
          def _update():
              import sqlite3
              try:
                  with sqlite3.connect(db_name, timeout=self.lru_timeout) as conn:
                      """ last_read is a result of time.time()"""
                      conn.execute('CREATE TABLE IF NOT EXISTS usage '
                                   '(hash VARCHAR(32), last_read FLOAT)')
                      conn.commit()
                      cur = conn.execute('select * from usage where hash=?', (hash_value,))
                      row = cur.fetchone()
                      if not row:
                          conn.execute("insert into usage(hash, last_read) values(?, ?)", (hash_value, time.time()))
                      else:
                          conn.execute("update usage set last_read=? where hash=?", (time.time(), hash_value))
                      conn.commit()
              except sqlite3.OperationalError:
                  # if there are many jobs to write to same database at same time, the timeout could be hit
                  logger.debug('could not update LRU info for db %s', db_name)
  
          # this could lead to another (rare) race condition during cleaning...
          #import threading
          #threading.Thread(target=_update).start()
          _update()
docs: timestamps are being stored distributed over several lru databases.
          The timestamp is a time.time() snapshot (float), which are seconds since epoch.