Up until now, you have seen how to handle locking, as well as some basic concurrency. In this section, you will learn about transaction isolation. To me, this is one of the most neglected topics in modern software development. Only a small fraction of software developers are actually aware of this issue, which in turn leads to mind-boggling bugs.

Here is an example of what can happen:

Most users would actually expect the first transaction to always return 300, regardless of the second transaction. However, this isn't true. By default, PostgreSQL runs in the READ COMMITTED transaction isolation mode. This means that every statement inside a transaction will get a new snapshot of the data, which will be constant throughout the query.

If you want to avoid this, you can use TRANSACTION ISOLATION LEVEL REPEATABLE READ. In this transaction isolation level, a transaction will use the same snapshot through the entire transaction. Here's what will happen:

As we've outlined, the first transaction will freeze its snapshot of the data and provide us with constant results throughout the entire transaction. This feature is especially important if you want to run reports. The first and last page of a report should always be consistent and operate on the same data. Therefore, the repeatable read is key to consistent reports.

Note that isolation-related errors won't always pop up instantly. Sometimes, trouble is noticed years after an application has been moved to production.