The Rest is History: Investigations of WAL History Files

PostgreSQL uses the concept of a timeline to identify a series of WAL records in space and time. Each timeline is identified by a number, a decimal in some places, hexadecimal in others. Each time a database is recovered using point in time recovery and sometimes during standby/replica promotion, a new timeline is generated.

A common mistake is to assume that a higher timeline number is synonymous with the most recent data. While the highest timeline points to the latest incarnation of the database, it doesn't guarantee that the database indeed holds the most useful data from an application standpoint. To discern the validity of this statement, a closer examination of the Write-Ahead Logging (WAL) history files is essential, unraveling the messages they convey.

In this discussion, we will explore a recovered database and trace the narrative embedded in the history files. By the conclusion, you will have gained a deeper insight into the functionality of these history files within Postgres, empowering you to address queries related to recovery processes and the database's historical journey (or may I call it the 'family tree').

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