Nobody likes slow applications. Long waits for pages to load frustrate users, so it is important to detect and correct performance problems as soon as they appear. In this chapter, two important performance aspects of web applications are considered.
When application performance slowly degenerates with time, it is likely due to slow database queries, which get worse as the size of the database grows. Optimizing database queries can be as simple as adding more indexes or as complex as adding a cache between the application and the database. The
explain statement, available in most database query languages, shows the steps the database takes to execute a given query, often exposing inefficiencies in database or index design.
But before starting to optimize queries, it is necessary to determine which queries are the ones that are worth optimizing. During a typical request several database queries may be issued, so it is often hard to identify which of all the queries are the slow ones. Flask-SQLAlchemy has an option to record statistics about database queries issued during a request. In Example 16-1 you can see how this feature can be used to log queries that are slower than a configured threshold.