There's a good blog article on the Xooglers (ex-Googlers) web site about how they use MySQL to run their adwords program. If you're not familiar, adwords generates something like 99.9% of Google's advertising revenues, so it certainly a business critical application. While Google is generally quite secretive about their internal operations, there are some interesting tidbits in this posting about how Google tried a commercial, closed-source database but it was never able to perform as well as MySQL. A few select quotes:
- "AdWords was built using the MySQL database, which is open-source and therefore available for free. It is by now also nearly as full-featured as the best commercial databases, but back in 2000 this was not the case."
- "Many managers seem to have this idea that it is invariably true that you get what you pay for, and that therefore nothing that is available for free can possibly be any good. Using MySQL was acceptable as an expedient to get things up and running quickly and with a minimal of capital outlay, but now that things were settling down it was time to recognize that this was really, fundamentally, a mistake, and it should be fixed sooner rather than later."
- "We finally decided to go with a commercial database (I won't say which one) over the objections of a number of engineers, including myself. To ease the transition it was decided to convert AdWords over to the new system first, and to do the main ads system later."
- "To make a long story short, it was an unmitigated disaster. The new system was slower than molasses in February. Some heroic optimization efforts eventually produced acceptable performance, but it was never as good as the old MySQL-based system had been."
- "I have heard through the grapevine that they eventually went back to MySQL. (Since then, MySQL has added many of the features that had been missing at the time.)"
It's a good article and provides some insight into the thinking around open source from a user's perspective. Here's a couple of other interesting links.