data_corrupted could not access status of transaction N — XX001

Symptoms

PostgreSQL could not read the commit status (from the clog / pg_xact)">pg_xact/clog area) for a transaction id it needed. This points to corruption of transaction-status data and raises SQLSTATE XX001 (data_corrupted).

• A DETAIL usually names a missing/short clog file.
• Indicates damage to pg_xact (commit log) or related SLRU data.
• May surface during reads, vacuum, or visibility checks.

What the server log shows

ERROR:  could not access status of transaction 1234567
DETAIL:  Could not open file "pg_xact/0001": No such file or directory.

Why PostgreSQL raises this — what the manual says

As Section 66.1 Database File Layout explains:

PostgreSQL could not read a transaction’s commit/abort status from the pg_xact (transaction commit log) subdirectory under the data directory — typically a symptom of data corruption or a missing or truncated commit-log segment; investigate disk integrity and recent crashes before continuing.

To decide tuple visibility, PostgreSQL must look up each transaction’s commit status in pg_xact. If that data is missing or unreadable, visibility cannot be determined and it reports XX001.

Common causes

• Corruption or loss of pg_xact/clog files.
• A truncated or improperly restored data directory.
• Transaction-id wraparound damage or storage faults.

How to fix it

1. Back up the data directory before attempting recovery.
2. Restore from a consistent backup/PITR — the most reliable fix.
3. For wraparound issues, ensure aggressive vacuuming; advanced clog repair only under expert guidance (risk of data loss).

Related & next steps

• SQLSTATE XX001 — Data Corrupted (full reference)
• Message: database is not accepting commands to avoid wraparound data loss
• Glossary: Transaction ID

Reference: PostgreSQL 18 Section 76.1 “Database File Layout”.