Re: Change sort order on UUIDs?

On Thu, Jun 14, 2007 at 03:38:44PM -0400, Robert Wojciechowski wrote:
> I've been testing the new UUID functionality in 8.3dev and noticed that
> UUIDs are sorted using memcmp in their default in-memory layout, which
> is:
>      struct uuid {
>              uint32_t        time_low;
>              uint16_t        time_mid;
>              uint16_t        time_hi_and_version;
>              uint8_t         clock_seq_hi_and_reserved;
>              uint8_t         clock_seq_low;
>              uint8_t         node[_UUID_NODE_LEN];
>      };
> When done that way, you're going to see a lot of index B-tree
> fragmentation with even DCE 1.1 (ISO/IEC 11578:1996) time based UUIDs,
> as described above. With random (version 4) or hashed based (version 3
> or 5) UUIDs there's nothing that can be done to improve the situation,
> obviously.

I suggest that treating the UUID as anything other than a unique
random value is a mistake. There should be no assumptions by users
with regard to how the order is displayed. Also, as UUID generation
based on time is always in sequence, it seems to me that sorting by
UUID time would have the effect of inserts always being to the end of
the index. While this might pack tightly, wouldn't this hurt
concurrency? Random access vs sequential performance. For UUID, I
would value random access before sequential performance. Why would
anybody scan UUID through the index in "sequential" order?

Cheers,
mark

-- 
mark@mielke.cc / markm@ncf.ca / markm@nortel.com     __________________________
.  .  _  ._  . .   .__    .  . ._. .__ .   . . .__  | Neighbourhood Coder
|\/| |_| |_| |/    |_     |\/|  |  |_  |   |/  |_   | 
|  | | | | \ | \   |__ .  |  | .|. |__ |__ | \ |__  | Ottawa, Ontario, Canada
 One ring to rule them all, one ring to find them, one ring to bring them all                      and in the darkness
bindthem...
 
                          http://mark.mielke.cc/