What is a better approach when it comes to performance and optimal resource utilization: accessing a database multiple times through AJAX to only get the exact information needed when it is needed, or performing one access to retrieve an object that holds all information that might be needed, with a high probability that not all is actually needed?

I know how to benchmark the actual queries, but I don't know how to test what is best when it comes to database performance when thousands of users are accessing the database simultaneously and how connection pooling comes into play.