SQL Server and UUIDs

# Introduction

When designing database tables to maintain the state of your objects you often have entities that do not have a Natural Key (attributes that exist on the entity itself) to uniquely identify them over time. One example can be a Customer:

Name Email Address
Jane Doe jane.doe@example.com Merrick, NY 11566

Any of the attributes can change. That doesn’t mean that the Customer becomes a different person. If Jane Doe is married and moves to a new state, she is still the same customer:

Name Email Address
Jane Smith jane.smith@example.com Philadelphia, PA 19111

To identify that entity over time we define a Surrogate Key. A proxy representing the entity.

ID Name Email Address
12 Jane Smith jane.smith@example.com Philadelphia, PA 19111

There are some practical concerns though when choosing an appropriate key. What format should it take? Are there implementation concerns?

# Choosing a key format

A simple integer seems reasonable at first but depending on the volume of data you can have problems if it’s not a big enough. Database systems give you many choices in this area and it can be easy to make the wrong one. Choosing an integer as a surrogate key (int32) means you’re limited to 232 = ~4 billion records max. Also, if you move your record to another table or another database, you can have collisions if they are using the same scheme.

You might recall when the Social Media platform Parler crashed:

Parler Crash

Credit: @saramehmei

In their case they chose a signed integer (int31) as a key which meant they were limited to 231 = ~2.1 billion records max.

A seemingly reasonable alternative solution is to use a UUID/GUID as the Surrogate key.

This poses two challenges though:

  1. A UUID is a big key: 128 bits (16 bytes)
  2. It’s pseudo-random and therefore unsorted

The first point is not so bad as a problem but the second point can be a significant challenge.

With an unordered key the database can’t store records on disk in a useful way. Which means if there is a need to search for records and they aren’t in memory a terrible amount of work (I/O operations) need to be done to find the appropriate records. A weak analogy: It’s easy for me to store and find the records of everyone when I can group them by the first letter of their last name. It’s not that easy if a random number identifies them instead.

So can do we get the benefits of clustering along with the benefits of a UUIDs uniqueness? In other words: is it possible to have a random, unique, but also ordered key?

The answer is yes, but you must choose the correct type of UUID

# Comparing UUIDs

UUIDs have the following format and you can determine the type you’re dealing with easily:

   A      B     C    D       E
  • M - version
  • N – variant


Version Description
0 nil UUID 00000000-0000-0000-0000-000000000000
1 data-time + MAC address (utilizes a Gregorian epoch timestamp)
2 date-time + MAC Address + DCE Security (utilizes a Gregorian epoch timestamp)
3 namespace name based (MD5)
4 Random
5 namespace name based (SHA-1)

So given the choices above which are the standard types it looks like we can’t use any of them as the best candidates (1 and 2) rely on the MAC of the underlying machine. If I’m on a virtual machine what does that mean? There are other implications as well…

Luckily, there is a draft RFC for new versions of UUIDs:

Version Description
6 field-compatible version of UUID1, reordered for improved DB locality (utilizes a Gregorian epoch timestamp)
7 Time ordered based on Unix Time epoch. Should be used instead of UUID1 and UUID6 when possible
8 Experimental and vendor-specific use cases. Uniqueness is not assumed

So, versions 6 and 7 provide us a key with all the attributes we desire.

# Practical Implications

What does that mean practically? Can the database systems we use generate those for us or do we have to write an implementation ourselves in the application layer?

The answer is (mostly) yes. SQL Server provides a function called NEWSEQUENTIALID() which guarantees uniqueness, does not rely on a MAC address, and provides ordering (unless the database server fails and is restored on another machine within a short enough time period). This function does not generate one of the (draft) standard formats, but has similar properties.

Other major database systems have similar functionality, or will shortly. I’ll focus on SQL Server only in this post as it’s been in the top 3 of rankings for years and is well-known and commonly in use; and I only spent time looking into that implementation specifically.

A final note here of possible interest is Entity Framework. If you’re using Entity Framework the sequential UUIDs will be generated for you automatically if the following is true:

  1. You are using SQL Server and don’t specify a default value
  2. You leverage the Guid type
  3. You specify the .ValueGeneratedOnAdd() method in the fluent API


# References and Further Reading


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