A common use case, that can often bring a database to its knees, is reporting. Reports usually run on large amounts of data, and can take considerable time if poorly written.
If you already have a method for retrieving a single item from a DB, then it can be tempting to reuse this for reporting purposes and for calculating metrics inside the application. However, this is best avoided and reports should generally have their own dedicated queries.
Even better is to have another database, dedicated to reporting, which is populated from the main DB or from backups. This way, intensive reports won't affect the performance of the main application at the expense of not including real-time data. This is sometimes known as a data warehouse ...
