Question

How are GUIDs generated in SQL Server?

I understand that I should use newid(), but what is the algorithm that function uses? Is it a hash of the time/date?

Answer 1

The algorithm to create it is described here: http://en.wikipedia.org/wiki/Globally_Unique_Identifier

Algorithm

In the OSF-specified algorithm for generating new (V1) GUIDs, the
user’s network card MAC address is used as a base for the last group
of GUID digits, which means, for example, that a document can be
tracked back to the computer that created it. This privacy hole was
used when locating the creator of the Melissa worm[2]. Most of the
other digits are based on the time while generating the GUID. V1 GUIDs
which contain a MAC address and time can be identified by the digit
“1” in the first position of the third group of digits, for example
{2f1e4fc0-81fd-11da-9156-00036a0f876a}. V4 GUIDs use the later
algorithm, which is a pseudo-random number. These have a “4” in the
same position, for example {38a52be4-9352-453e-af97-5c3b448652f0}.
More specifically, the ‘data3’ bit pattern would be 0001xxxxxxxxxxxx
in the first case, and 0100xxxxxxxxxxxx in the second. Cryptanalysis
of the WinAPI GUID generator shows that, since the sequence of V4
GUIDs is pseudo-random; given full knowledge of the internal state, it
is possible to predict previous and subsequent values