This is old news, but not an old concept. Google uses MySQL on a number of their products, but those products require some features that do not exist in MySQL in its current state. Rather than waiting 2 years, to pay umpteenthousnds of dollars for an upgrade, because MySQL is opensource, Google just writes their own additions to the package. And since MySQL was open-source to begin with, why not open-source their changes and let anyone have them? That’s exactly what they did:

We think MySQL is a fantastic data storage solution, and as our projects push the requirements for the database in certain areas, we’ve made changes to enhance MySQL itself, mainly in the areas of high availability and manageability.

[Via Google Code]

We use MySQL on a number of our web products:

• The Support Wiki
• CSP magazine
• FreshlySqueezed
• blogs.csp.edu
• Our internal support tracking
• A number of internal statistics and availability applications

Why?

• MySQL is free
• MySQL is highly scaleable
• MySQL backups take seconds to execute or restore
• MySQL runs on linux, mac, or windows servers
• MySQL’s server requirements are minimal, to the point of being a joke (4 MB disk space; 1 MB Ram)
• Did I mention it’s free?