Q:

When do we need high memory and CPU in amazon AWS RDS?

I understand that RDS instances will potentially require high disk space as the business grow. However, when does RDS require high memory? I thought high memory will be required in EC2 where a lot of business logic happens, but not necessarily the case in RDS. I understand i probably missing something here and help would be appreciated. Thanks!

A:

This is really a question of how databases use memory. 
When a query engine is executing a statement, there are times when memory is used to store transient structures like spools, table variables, and so on. Under a lot of memory pressure, this stuff often ends up being written to tempdb (in the case of SQL server), which adds i/o overhead.
For a specific case, you really need to look at your own usage profiles to see if extra memory would help. 

