correct, PostgreSQL is recommended for VoIP-VSA-H323 accounting, than Mysql. That fact however does not stop a lot of billing software packages (like DataTechLabs, Alepo RBS and etc) to use Mysql for VoIP billing.
The below link is also a great help for those who have Radius experience and the intention to start working on H323 auth/acc software.
http://www.cisco.com/warp/public/cc/so/cuso/sp/sms/acct/caaaf_cg.htm