How are WAL segments managed and reused?

WAL (Write-Ahead Logging) segments are parts of the transaction log used to ensure database reliability and recovery. Management and reuse of WAL segments occur as follows:

• WAL segments are created sequentially and filled with transaction records.
• When the current segment is filled, a new segment is created, and recording continues in it.
• After the data from WAL has been successfully applied to the main database (for example, after a checkpoint), old WAL segments are no longer needed for recovery.
• These old segments can be reused by the system, i.e., overwritten with new data, to avoid creating an infinite number of files.

Thus, managing WAL segments involves cyclic use of a limited set of files, which saves disk space and maintains performance. In PostgreSQL, this is implemented through parameters like wal_keep_segments and checkpoints, which determine which segments can be safely reused.