Was Data Updated by Mistake?
Accidental Data Update Recovery
Scenario Description
An UPDATE statement with incorrect WHERE conditions overwrites thousands of rows of important data, or a batch job writes wrong values to the database. The original values before the update are needed, but no backup exists from before the incident.
Typical scenarios include:
- UPDATE SET column = value without WHERE clause, affecting entire table
- Wrong WHERE clause updating incorrect data
- ETL jobs writing incorrect data to production tables
- Data migration scripts overwriting correct data
- Application logic errors causing data overwrites
Challenges
UPDATE operations in PostgreSQL overwrite existing data. Once committed, the previous values are no longer accessible through SQL queries. Finding what the data looked like before the update is extremely difficult.
Why UPDATE is Hard to Recover
- - Original values directly overwritten
- - Cannot query old data via SQL
- - Traditional backup recovery is costly
- - Scope of impact can be massive
Difference from DELETE
- - DELETE completely removes rows
- - UPDATE modifies specific columns
- - Both require WAL-based recovery
- - PDU can handle both cleanly
PDU Solution
PDU leverages WAL archives to recover pre-update values. Every UPDATE in PostgreSQL is logged in WAL with enough information to reconstruct the original row. PDU parses these records to extract what the data looked like before the problematic UPDATE.
Recovery Principle
PDU Core Capabilities
- Extracts original row values from before UPDATE operations in WAL
- Identifies the exact timestamp and transaction of the unwanted changes
- Allows selective recovery of specific columns or rows
- Generates SQL statements to restore data to its previous state
- Works even when the UPDATE affected millions of rows
- Supports both time range and transaction-based recovery
DELETE vs UPDATE Recovery Comparison
| Aspect | Deleted Records | Updated Records |
|---|---|---|
| Recovery Type Setting | p restype delete; | p restype update; |
| Restore Command | restore del all; | restore upd all; |
| What is Recovered | Complete deleted rows | Original values before update |
Common Use Cases
Accidental Mass UPDATE
When a WHERE clause was missing or incorrect, causing unintended rows to be updated. Use Time Range Mode to recover all affected rows.
Specific Transaction Rollback
When you need to undo changes from a specific application operation. Use Transaction Mode to target that exact transaction.
Data Auditing
When you need to see what values existed before certain changes were made, for compliance or debugging purposes.
Prerequisites
- WAL archiving must be enabled
The principle is the same as deleted records recovery - extracting data from WAL files.
- Ensure WAL files are archived after the update
Run after accidental update:
SELECT pg_switch_wal(); CHECKPOINT;