The Ultimate Guide to PostgreSQL WAL Inspection with pg_walinspect

PostgreSQL’s Write-Ahead Logging (WAL) is essential for data durability, crash recovery, replication, point-in-time recovery (PITR), and logical decoding. WAL files record every database change before it's written to data files, ensuring the system can restore consistency or propagate changes to replicas. Direct inspection of these log records wasn’t possible from SQL before PostgreSQL 14, but the introduction of the pg_walinspect extension makes the internal workings of WAL transparent and accessible for in-depth analysis.

What Sets pg_walinspect Apart?

pg_walinspect is a SQL-based extension for exploring WAL records within PostgreSQL (v14+). Unlike command-line tools such as pg_waldump, pg_walinspect lets you analyze WAL content directly in SQL, making it easy to embed log analytics in your dashboards, CI/CD pipelines, and monitoring systems.

Why Inspect WAL?

• Troubleshoot replication and recovery issues quickly.
• Monitor transaction patterns and volume.
• Audit changes or backtrace data for forensic analysis.
• Prepare for PITR by identifying required WAL segment boundaries.
• Perform performance analysis by measuring transaction rates and bottlenecks.

Installing and Enabling pg_walinspect

-- Connect as a superuser and enable the extension
CREATE EXTENSION pg_walinspect;

Verify installation:

SELECT * FROM pg_available_extensions WHERE name = 'pg_walinspect';

List extension functions:

\dx+ pg_walinspect

Sample output highlights functions such as:

• pg_get_wal_block_info(pg_lsn, pg_lsn, boolean)
• pg_get_wal_record_info(pg_lsn)
• pg_get_wal_records_info(pg_lsn, pg_lsn)
• pg_get_wal_stats(pg_lsn, pg_lsn, boolean)percona+1

Example Queries: Uncovering WAL Secrets

1. Viewing WAL Statistics

Get a timeline of WAL stats:

SELECT * FROM pg_get_wal_stats('0/0', NULL);

Explanation of parameters:

• '0/0' – This indicates the starting Log Sequence Number (LSN). Using 0/0 tells PostgreSQL to start from the earliest WAL record available.
• NULL – This tells PostgreSQL to continue scanning until the latest WAL record.

What this function returns:

• resource_manager/record_type – The type of WAL record, grouped by resource manager.
• count – How many times that record type appears in the scanned range.
• count_percentage – The proportion of that record type relative to all scanned records.
• record_size – Total size (in bytes) of the WAL records for that type.
• record_size_percentage – Percentage of WAL space taken up by that record type.
• fpi_size – Bytes consumed by Full Page Images (FPIs) for that record type.
• fpi_size_percentage – Percentage of WAL taken by FPIs for that type.
• combined_size – Sum of record_size and fpi_size.
• combined_size_percentage – Overall percentage of WAL space occupied by that type.

2. Find Current WAL Position

SELECT pg_current_wal_lsn(), pg_walfile_name(pg_current_wal_lsn());

Explanation of functions:

• pg_current_wal_lsn() – Returns the current WAL Log Sequence Number (LSN) where the next record will be written.
• pg_walfile_name(lsn) – Given an LSN, this returns the name of the WAL segment file containing it.

You should see a result like this

 pg_current_wal_lsn |     pg_walfile_name      
--------------------+--------------------------
 432/83C2BF58       | 000000010000043200000083

How to read this:

• 432/83C2BF58 – This is the current LSN in hexadecimal form. The first part (432) is the log ID, and the second part (83C2BF58) is the byte offset within that log.
• 000000010000043200000083 – This is the WAL segment file name.
  • The first 8 digits (00000001) represent the timeline.
  • The next 8 digits (00000432) are the log ID in hexadecimal.
  • The final 8 digits (00000083) are the segment number.

3. Reading WAL Records

SELECT * FROM pg_get_wal_records_info('0/0', NULL) LIMIT 10;

Explanation of parameters:

• '0/0' – Start from the very first WAL record available.
• NULL – Continue reading until the latest record.
• LIMIT 10 – Only show the first 10 records for easier inspection.

What this function returns:

• lsn – The exact Log Sequence Number of the record.
• prev_lsn – The LSN of the previous WAL record.
• xid – The transaction ID associated with this record (if any).
• timestamp – The time the change was logged.
• resource_manager – The PostgreSQL component responsible for this record (e.g., Heap, Btree, XLOG, etc.).
• record_type – A more specific operation type handled by the resource manager.
• record_length – Size of the record in bytes.
• fpi_length – Size of any Full Page Images stored for this change.
• description – A textual explanation of what this record does (e.g., "insert", "delete", "update").

4. Inspecting Specific WAL Ranges

Grab records between two LSN values:

SELECT * FROM pg_get_wal_records_info('0/163840', '0/200000');

5. Block-Level Change Analysis

Track block-level changes (physical changes to individual data blocks):

SELECT * FROM pg_get_wal_block_info('0/0', NULL, true) LIMIT 10;

Returns details about changed blocks, tablespace, database IDs, relation types, forkname, and specific block references, enabling granular change tracking.postgresql

6. WAL File Management and Replication Lag

Calculate replication lag:

SELECT pg_current_wal_lsn() - pg_last_wal_replay_lsn();

Monitor WAL archiving status:

SELECT * FROM pg_stat_archiver;

Monitor WAL streaming replication:

SELECT * FROM pg_stat_replication;

7. Summarize WAL Activity for PITR or Auditing

Correlate specific transactions or events with WAL activity to estimate volume:

SELECT wal_records, wal_bytes, wal_fpi FROM pg_stat_wal;

Useful for auditing or tuning wal_keep_size for replication retention.

Beyond pg_walinspect: Other Views

• pg_stat_wal (PostgreSQL 14+): Live WAL statistics including counters for WAL records, bytes, FPIs
• pg_stat_bgwriter: Background writing activity and checkpoints.
• pg_stat_archiver: WAL archiving progress.
• pg_stat_replication: Streaming replication status with WAL LSNs for master and replica nodes.

Practical Use Cases

• Auditing database changes with detailed block-level insights.
• Debugging replication by tracking WAL activity and lag between primary and standby.
• Estimating WAL volume for backup/replication retention policies.
• PITR preparation by examining WAL segment boundaries.
• Change rate analysis to pinpoint peak loads or bottlenecks.

Limitations & Best Practices

• Superuser privileges required for WAL inspection.
• Can only inspect WAL segments currently present—not archived or deleted files.
• pg_walinspect shows structural record info, not SQL statements.
• For advanced audit or logical changes, consider logical decoding (e.g., pg_logical, pgoutput) for reconstructing SQL.postgresql

Conclusion

The pg_walinspect extension transforms PostgreSQL’s Write-Ahead Log from an opaque internal mechanism into a rich, queryable source of operational insight. By enabling direct SQL access to WAL records, it empowers administrators and developers to troubleshoot replication, analyze workload patterns, prepare for PITR, and audit changes with precision—all without leaving the database environment. While it requires superuser access and only works with available WAL segments, its ability to surface granular, structured information makes it an indispensable tool for performance tuning, disaster recovery planning, and deep operational diagnostics in PostgreSQL.