Does AWS RDS support MEMORY storage engine?
This answer applies to AWS Aurora not straight MySQL RDS.

Just because SHOW ENGINES says the database has the MEMORY storage engine doesn't mean you can actually use it. We have found that in AWS Aurora the SHOW ENGINES; call will tell you that the MEMORY engine is supported. However, if you try to change the engine to MEMORY or create a non-temporary table with MEMORY engine you'll get an InnoDB engine, because Aurora only supports InnoDB. When you run the engine change you'll get a warning that the InnoDB engine has been used.

Here's what I got from AWS support:


  Aurora is designed to provide support for only InnoDB storage engine for regular tables. Engines other than InnoDB are only supported for session-level temporary tables. With default configuration, when you try to create a table using a storage engine other than InnoDB, Aurora will implicitly create the table as InnoDB. Session level temporary tables are not persistent and they will be dropped once the session is completed.