What are the reasons *not* to use a GUID for a primary key?

Question

Whenever I design a database I automatically start with an auto-generating GUID primary key for each of my tables (excepting look-up tables)

I know I\'ll never lose

Answer 1

A potentially big reason, but one often not thought of, is if you might have to provide compatibility with an Oracle database in the future.

Since Oracle doesn't have a uniqueid column data type, it can lead to a bit of a nightmare when you have two different data types for the same primary key across two different databases, especially when an ORM is involved.