uuid

http://dinglin.iteye.com/blog/1487537

http://www.cnblogs.com/jdonson/archive/2009/07/22/1528466.html

http://blog.chinaunix.net/uid-26495963-id-3150576.html

 http://www.ietf.org/rfc/rfc4122.txt

http://blog.csdn.net/sasoritattoo/article/details/10365847

mysql5.7

uuid 源代码:

String *Item_func_uuid::val_str(String *str)
{
  DBUG_ASSERT(fixed == 1);
  char *s;
  THD *thd= current_thd;

  mysql_mutex_lock(&LOCK_uuid_generator);
  if (! uuid_time) /* first UUID() call. initializing data */
  {
    ulong tmp=sql_rnd_with_mutex();
    uchar mac[6];
    int i;
    if (my_gethwaddr(mac))
    {
      /* purecov: begin inspected */
      /*
        generating random "hardware addr"
        and because specs explicitly specify that it should NOT correlate
        with a clock_seq value (initialized random below), we use a separate
        randominit() here
      */
      randominit(&uuid_rand, tmp + (ulong) thd, tmp + (ulong)global_query_id);
      for (i=0; i < (int)sizeof(mac); i++)
        mac[i]=(uchar)(my_rnd(&uuid_rand)*255);
      /* purecov: end */    
    }
    s=clock_seq_and_node_str+sizeof(clock_seq_and_node_str)-1;
    for (i=sizeof(mac)-1 ; i>=0 ; i--)
    {
      *--s=_dig_vec_lower[mac[i] & 15];
      *--s=_dig_vec_lower[mac[i] >> 4];
    }
    randominit(&uuid_rand, tmp + (ulong) server_start_time,
           tmp + (ulong) thd->status_var.bytes_sent);
    set_clock_seq_str();
  }

  ulonglong tv= my_getsystime() + UUID_TIME_OFFSET + nanoseq;

  if (likely(tv > uuid_time))
  {
    /*
      Current time is ahead of last timestamp, as it should be.
      If we "borrowed time", give it back, just as long as we
      stay ahead of the previous timestamp.
    */
    if (nanoseq)
    {
      DBUG_ASSERT((tv > uuid_time) && (nanoseq > 0));
      /*
        -1 so we won't make tv= uuid_time for nanoseq >= (tv - uuid_time)
      */
      ulong delta= min<ulong>(nanoseq, (ulong) (tv - uuid_time -1));
      tv-= delta;
      nanoseq-= delta;
    }
  }
  else
  {
    if (unlikely(tv == uuid_time))
    {
      /*
        For low-res system clocks. If several requests for UUIDs
        end up on the same tick, we add a nano-second to make them
        different.
        ( current_timestamp + nanoseq * calls_in_this_period )
        may end up > next_timestamp; this is OK. Nonetheless, we'll
        try to unwind nanoseq when we get a chance to.
        If nanoseq overflows, we'll start over with a new numberspace
        (so the if() below is needed so we can avoid the ++tv and thus
        match the follow-up if() if nanoseq overflows!).
      */
      if (likely(++nanoseq))
        ++tv;
    }

    if (unlikely(tv <= uuid_time))
    {
      /*
        If the admin changes the system clock (or due to Daylight
        Saving Time), the system clock may be turned *back* so we
        go through a period once more for which we already gave out
        UUIDs.  To avoid duplicate UUIDs despite potentially identical
        times, we make a new random component.
        We also come here if the nanoseq "borrowing" overflows.
        In either case, we throw away any nanoseq borrowing since it's
        irrelevant in the new numberspace.
      */
      set_clock_seq_str();
      tv= my_getsystime() + UUID_TIME_OFFSET;
      nanoseq= 0;
      DBUG_PRINT("uuid",("making new numberspace"));
    }
  }

  uuid_time=tv;
  mysql_mutex_unlock(&LOCK_uuid_generator);

  uint32 time_low=            (uint32) (tv & 0xFFFFFFFF);
  uint16 time_mid=            (uint16) ((tv >> 32) & 0xFFFF);
  uint16 time_hi_and_version= (uint16) ((tv >> 48) | UUID_VERSION);

  str->realloc(UUID_LENGTH+1);
  str->length(UUID_LENGTH);
  str->set_charset(system_charset_info);
  s=(char *) str->ptr();
  s[8]=s[13]='-';
  tohex(s, time_low, 8);
  tohex(s+9, time_mid, 4);
  tohex(s+14, time_hi_and_version, 4);
  strmov(s+18, clock_seq_and_node_str);
  DBUG_EXECUTE_IF("force_fake_uuid",
                  strmov(s, "a2d00942-b69c-11e4-a696-0020ff6fcbe6");
                  );
  return str;
}