How PostgreSQL handles persistency and consistency: WALs
In the previous sections, you have seen how to interact with transactions and, most notably, how PostgreSQL executes every statement within a transaction, either explicitly or implicitly.
PostgreSQL does, internally, very complex work to ensure that consolidated data in storage reflects the status of the committed transactions. In other words, data can be considered consolidated only if the transaction that produced (or modified) it has been successfully committed. But this also means that, once a transaction has been successfully committed, its data is “safe” on storage, no matter what happens in the future: if a transaction is reported to be successful, its data must be made persistent, even if the database or the whole system crashes.
PostgreSQL manages transactions and data consolidations by means of WALs. This section introduces you to the concept of WALs and their use within PostgreSQL.
