The Price for GUIDs

What's the price you pay for using GUIDs as primary keys in databases? Many engineers choose GUIDs (Globally Unique Identifiers) as primary keys in databases because they ensure uniqueness across distributed systems. While GUIDs solve some architectural challenges, they also introduce hidden performance trade-offs that can significantly impact your database’s efficiency.

Here are a few challenges with GUIDs:

1. Index Fragmentation & Slower Query Performance Unlike sequential keys (INT, BIGINT), GUIDs are random, meaning new records get inserted anywhere in the index, not at the end. This can lead to:
   i. Heavily fragmented indexes, slowing down queries.
   ii. Higher I/O operations, increasing database load.
2. Increased Storage & Memory Usage A GUID (UNIQUEIDENTIFIER) is 16 bytes, while an INT is only 4 bytes. Using GUID can lead to:
   i. Larger indexe: more disk space and less efficient caching.
   ii. Increased memory consumption for joins and lookups.
   iii. More expensive replication and backups in large datasets.
3. Higher Insert Overhead Because GUIDs are non-sequential, new records may require page splits when inserted into a clustered index. This increases:
   i. Write amplification (more disk writes).
   ii. Increased locking and contention in high-traffic databases.

What's the solution?
Use Sequential GUIDs (NEWSEQUENTIALID or COMB GUIDs).

1. SQL Server provides NEWSEQUENTIALID() to generate GUIDs that are partially ordered, reducing fragmentation. See attached image for reference
2. If you’re using MySQL, PostgreSQL, or another database that doesn’t support NEWSEQUENTIALID(), you can generate COMB GUIDs in C# by embedding a timestamp into the GUID.

In conclusion, while GUIDs can hurt performance due to fragmentation, storage bloat, and insert overhead, sequential GUIDs (COMB GUIDs) improve indexing while maintaining uniqueness. Ultimately, choosing the right primary key depends on your architecture’s needs.