I've worked on a number of database systems in the past where moving entries between databases would have been made a lot easier if all the database keys had been GUID / UUID values. I've considered going down this path a few times, but there's always a bit of uncertainty, especially around performance and un-read-out-over-the-phone-able URLs.
Has anyone worked extensively with GUIDs in a database? What advantages would I get by going that way, and what are the likely pitfalls?
Advantages:
Disadvantages:
Personally, I use them for most PK's in any system of a decent size, but I got "trained" on a system which was replicated all over the place, so we HAD to have them. YMMV.
I think the duplicate data thing is rubbish - you can get duplicate data however you do it. Surrogate keys are usually frowned upon where ever I've been working. We DO use the WordPress-like system though:
UPDATE: So this one gets +1'ed a lot, and I thought I should point out a big downside of GUID PK's: Clustered Indexes.
If you have a lot of records, and a clustered index on a GUID, your insert performance will SUCK, as you get inserts in random places in the list of items (that's the point), not at the end (which is quick).
So if you need insert performance, maybe use a auto-inc INT, and generate a GUID if you want to share it with someone else (e.g., showing it to a user in a URL).
example.com/35/old-and-busted just became example.com/35/new-hotness and you're app can just check the title and forward the user on with a 301.
Why doesn't anyone mention performance? When you have multiple joins, all based on these nasty GUIDs the performance will go through the floor, been there :(
@Matt Sheppard:
Say you have a table of customers. Surely you don't want a customer to exist in the table more than once, or lots of confusion will happen throughout your sales and logistics departments (especially if the multiple rows about the customer contain different information).
So you have a customer identifier which uniquely identifies the customer and you make sure that the identifier is known by the customer (in invoices), so that the customer and the customer service people have a common reference in case they need to communicate. To guarantee no duplicated customer records, you add a uniqueness-constraint to the table, either through a primary key on the customer identifier or via a NOT NULL + UNIQUE constraint on the customer identifier column.
Next, for some reason (which I can't think of), you are asked to add a GUID column to the customer table and make that the primary key. If the customer identifier column is now left without a uniqueness-guarantee, you are asking for future trouble throughout the organization because the GUIDs will always be unique.
Some "architect" might tell you that "oh, but we handle the real customer uniqueness constraint in our app tier!". Right. Fashion regarding that general purpose programming languages and (especially) middle tier frameworks changes all the time, and will generally never out-live your database. And there is a very good chance that you will at some point need to access the database without going through the present application. == Trouble. (But fortunately, you and the "architect" are long gone, so you will not be there to clean up the mess.) In other words: Do maintain obvious constraints in the database (and in other tiers, as well, if you have the time).
In other words: There may be good reasons to add GUID columns to tables, but please don't fall for the temptation to make that lower your ambitions for consistency within the real (==non-GUID) information.
The main advantages are that you can create unique id's without connecting to the database. And id's are globally unique so you can easilly combine data from different databases. These seem like small advantages but have saved me a lot of work in the past.
The main disadvantages are a bit more storage needed (not a problem on modern systems) and the id's are not really human readable. This can be a problem when debugging.
There are some performance problems like index fragmentation. But those are easilly solvable (comb guids by jimmy nillson: http://www.informit.com/articles/article.aspx?p=25862 )
Edit merged my two answers to this question
@Matt Sheppard I think he means that you can duplicate rows with different GUIDs as primary keys. This is an issue with any kind of surrogate key, not just GUIDs. And like he said it is easilly solved by adding meaningfull unique constraints to non-key columns. The alternative is to use a natural key and those have real problems..
GUIDs may cause you a lot of trouble in the future if they are used as "uniqifiers", letting duplicated data get into your tables. If you want to use GUIDs, please consider still maintaining UNIQUE-constraints on other column(s).
The Cost of GUIDs as Primary Keys (SQL Server 2000)
Myths, GUID vs. Autoincrement (MySQL 5)
This is realy what you want.
UUID Pros
GUID Cons
One other small issue to consider with using GUIDS as primary keys if you are also using that column as a clustered index (a relatively common practice). You are going to take a hit on insert because of the nature of a guid not begin sequential in anyway, thus their will be page splits, etc when you insert. Just something to consider if the system is going to have high IO...
There is one thing that is not really addressed, namely using random (UUIDv4) IDs as primary keys will harm the performance of the primary key index. It will happen whether or not your table is clustered around the key.
RDBMs usually ensure the uniqueness of the primary keys, and ensure the lookups by a key, in a structure called BTree, which is a search tree with a large branching factor (a binary search tree has branching factor of 2). Now, a sequential integer ID would cause the inserts to occur just one side of the tree, leaving most of the leaf nodes untouched. Adding random UUIDs will cause the insertions to split leaf nodes all over the index.
Likewise if the data stored is mostly temporal, it is often the case that the most recent data needs to be accessed and joined against the most. With random UUIDs the patterns will not benefit from this, and will hit more index rows, thereby needing more of the index pages in memory. With sequential IDs if the most-recent data is needed the most, the hot index pages would require less RAM.
Advantages:
One thing not mentioned so far: UUIDs make it much harder to profile data
For web apps at least, it's common to access a resource with the id in the url, like stackoverflow.com/questions/45399. If the id is an integer, this both
From the first point, I can combine the timestamp from the question and the number to profile how frequently questions are asked and how that changes over time. this matters less on a site like Stack Overflow, with publicly available information, but, depending on context, this may expose sensitive information.
For example, I am a company that offers customers a permissions gated portal. the address is portal.com/profile/{customerId}. If the id is an integer, you could profile the number of customers regardless of being able to see their information by querying for lastKnownCustomerCount + 1 regularly, and checking if the result is 404 - NotFound (customer does not exist) or 403 - Forbidden (customer does exist, but you do not have access to view).
UUIDs non-sequential nature mitigate these issues. This isn't a garunted to prevent profiling, but it's a start.
The biggest criticism against UUID as database keys is: inefficient indexing.
The arbitrary layout of bits within UUID values inhibit efficient indexing. This inefficiency is not significant with smaller sized tables, but becomes a major bottleneck as tables grow upwards of a million records.
This criticism is now resolved by the new Version 6 and Version 7 UUIDs.
In 2024, the specification for UUIDs was revamped in a big way. RFC 9562 obsoletes RFC 4122.
Firstly, RFC 9562 was entirely rewritten. The writing is much improved, easier to read, bringing clarity and insight. The new RFC discusses the pros and cons of the various Versions, explaining the motivations for the changes. And thankfully the new spec lends some guidance in choosing a Version. So I strongly recommend reading the actual specification.
Secondly, RFC 9562 defines three new Versions of UUID: Versions 6, 7, & 8.
Version 1 and Version 6 are similar, both being based on points in time and space.
Version 6 supplants Version 1, rearranging its bits for optimal indexing, and changing some of the semantics of its parts, while remaining field-compatible with Version 1.
👉🏽 For brownfield projects using Version 1, consider switching to Version 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time_high |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time_mid | ver | time_low |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| clock_seq | node |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| node |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version 7 is an all-new kind of UUID: time + random.
👉🏽 Version 7 is recommended for greenfield projects. Prefer 7 over 1 & 6 if possible.
Changes incude:
Implementations are free to choose to fill these otherwise-random bits with values based on sub-millisecond timestamp fraction, and/or a counter as defined in Versions 1 & 2. For example, Postgres 18 includes a 12-bit sub-millisecond timestamp fraction immediately after the timestamp, providing monotonicity for all UUIDv7 values generated by the same Postgres session (same backend process) (see post).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unix_ts_ms |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unix_ts_ms | ver | rand_a |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| rand_b |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rand_b |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+