data_corrupted unexpected chunk number N (expected N) for toast value N — XX001

Symptoms

While reassembling an out-of-line TOAST value, PostgreSQL found chunks in the wrong order. This indicates data corruption, so it raises SQLSTATE XX001 (data_corrupted).

• TOAST chunk numbers don’t match the expected sequence.
• Points to corruption in the TOAST table or its index.
• Reads of the affected wide value fail.

What the server log shows

ERROR:  unexpected chunk number 2 (expected 1) for toast value 24601 in pg_toast_16390

Why PostgreSQL raises this — what the manual says

Section 66.2.1 Out-of-Line, On-Disk TOAST Storage:

“Out-of-line values are divided (after compression if used) into chunks of at most TOAST_MAX_CHUNK_SIZE bytes (by default this value is chosen so that four chunk rows will fit on a page, making it about 2000 bytes).”

Large field values are split into numbered chunks in a TOAST table. If the chunk sequence is inconsistent (a chunk is misnumbered), the value cannot be reassembled and PostgreSQL reports XX001.

Common causes

• Storage-level corruption of the TOAST table or its index.
• Hardware faults, or crashes with disabled fsync.
• A corrupted TOAST index returning wrong rows.

How to fix it

1. REINDEX the TOAST index; if data itself is damaged, restore from backup.
2. Run hardware/filesystem checks; this signals real corruption.
3. Identify the affected row and consider deleting/repairing it after backing up.

Related & next steps

• SQLSTATE XX001 — Data Corrupted (full reference)
• Message: missing chunk number for toast value in pg_toast
• Glossary: TOAST

Reference: PostgreSQL 18 Section 73.2 “TOAST”.