In contrast to other systems TYPO3 always provided a database abstraction layer before anybody else in the industry even thought about it. Every TYPO3 developer knows $GLOBALS['TYPO3_DB'] by heart and keeps a steady love/hate relationship with it. This programming interface allows the CMS to run on database engines besides the native MySQL server, but it always came with a price tag: It was complex to run TYPO3 on anything beside MySQL under certain conditions and lots of time and money had to be put into it, especially for non-standard scenarios.
This is a companion discussion topic for the original entry at https://typo3.org/article/typo3-v81-tightening-the-screws/