We've checked with Adobe Support team and below is their response for "Is it recommended to use a PostgreSQL database with AEM? We are using AaaCS. In one of the community posts, it is mentioned that PostgreSQL is provided only for demo purposes. It will not be available for production environments. Please let us know if it is a recommended database or not. If not, please let us know the limitations and also, let us know the recommended RDBMS(MySQL,Microsoft SQL) to use with AaaCS.We have a requirement to retrieve data from an external database to display in our page. Do you have any recommended databases(MySQL,MSSQL) to use with AEMaaCS?".
1.There is no choice on persistence setup in AEMaaCS and in addition Adobe never supported RDBMS based persistence - those are only proof of concept solutions that are not production ready and don‘t satisfy our operational requirements for productive environments
2.If you have a requirement to retrieve data from an external database to display on your page then the best approach is to expose those data as APIs. You can expose them as REST APIs which can be consumed with AEM backend java or frontend script (js, angular, react) code based on needs.
