Locally unique identifier

Question

Question: When you have a .NET GUID for inserting in a database, it's structure is like this:

60 bits of timestamp, 
48 bits of computer identifier,
14 bits of uniquifier, and
 6 bits are fixed, 
----
128 bits total

Now I have problem with a GUID, because it's a 128 bit number, and some of the DBs I'm using only support 64 bit numbers.

Now I don't want to solve the dilemma by using an autoincrement bigint value, since I want to be able to do offline replication.

So I got the idea of creating a locally unique identifier class, which is basically a GUID downsized to a 64 bit value.

I came up with this:

day  9 bit (12*31=372 d)
year 8 bit (2266-2010 = 256 y)
seconds  17 bit (24*60*60=86400 s)
hostname 12 bit (2^12=4096)
random 18 bit (2^18=262144)
------------------------
          64 bits total

My question now is: The timestamp is pretty much fixed at 34 bits, leaving me with 64-34=30 bits for the hostname + random number.

Now my question: 1) Would you rather increase the hostname-hash bitsize and decrease the random bitsize, or increase the random bitsize and decrease the hostname-hash bitsize.

2) Exists there a hash algorithm that reduces every string to n-Bits? n being ideally = 12 or as near as possible.

Answer 1

Actually, .NET-generated GUIDs are 6 fixed bits and 122 bits of randomness.

You could consider just using 64 bits of randomness, with an increased chance of collision due to the smaller bit length. It would work better than a hash.

Answer 2

If space isn't a concern, then why don't you just use 2 columns that are 64bits wide, then split the guid in half using 8bytes for each then just convert those to your 64bit numbers and store it in 2 columns, then if you ever do need to upsize to another system, you'll still be unique you'll just need to factor in the rejoining of the 2 columns.

Answer 3

Why write your own? Why not just generate a uniformly random number? It will do the job nicely. Just grab the first X digits where X is whatever size you want... say 64-bits.

See here for info about RAND() vs. NEWID() in SQL Server, which is really just an indictment of GUIDs vs. random number generators. Also, see here if you need something more random than System.Random.