Understanding GUIDs and the (Nearly) Impossibility of Collisions in SQL Server

GUIDs, also known as UUIDs (Universally Unique Identifiers), are 128-bit values meant to be unique across systems. They are often used as primary keys in databases to ensure each record has a distinct identifier.

How are GUIDs generated in SQL Server?

SQL Server uses the NEWID() function to generate version 4 GUIDs. These are pseudo-random values, meaning they are created using an algorithm that appears random but is actually deterministic (repeatable given the same initial conditions).

Why are collisions possible?

While highly improbable, collisions can occur due to:

• Pure chance: Although the probability is incredibly low (estimated at 1 in 2^122, which is a number larger than the estimated number of atoms in the observable universe), two random GUIDs could theoretically be identical.
• Seeding issues: If the random number generator used by NEWID() is not properly seeded with unique values, it might generate the same sequence of GUIDs under specific circumstances. This is extremely rare but not impossible.

Sample Code:

-- Generate two GUIDs
DECLARE @guid1 uniqueidentifier, @guid2 uniqueidentifier;
SET @guid1 = NEWID();
SET @guid2 = NEWID();

-- Compare the GUIDs
SELECT @guid1, @guid2, 
       CASE WHEN @guid1 = @guid2 THEN 'Collision!' ELSE 'No collision.' END AS Result;

Related Issues and Solutions:

While collisions are highly unlikely, it's important to be aware of related issues and potential solutions:

• Duplicate key errors: If a collision occurs, you might encounter duplicate key errors when trying to insert new records with the same GUID.
• Solution: Implement checks before inserting new records. You can use a UNIQUE constraint on the GUID column or check for existing GUIDs before inserting with a query like SELECT COUNT(*) FROM YourTable WHERE MyGUID = @guid.
• Sequential GUIDs: If you require absolute uniqueness and cannot tolerate even the remote possibility of a collision, consider using NEWSEQUENTIALID() instead of NEWID(). However, this function is not guaranteed to be unique across multiple servers.