Is it safe to remove all data from the execution_data table in PostgreSQL to free up storage?

Below is some advice based on PostgreSQL experience and best practices:

Considerations Before Deleting Data

• Backup First:
  Make sure you have a recent backup (or have exported the data you might need later). Once you remove 31GB from your cluster, recovery could be difficult if the deletion turns out to be premature.

• Check Dependencies:
  Verify that no dependent objects (such as foreign key constraints, materialized views, or application code) rely on the data in the table. Removing data that is still in use may cause applications or reporting jobs to fail.

• Deletion Method Matters:

  • TRUNCATE vs. DELETE:
    If you want to remove all rows and there are no dependency conflicts, using TRUNCATE execution_data; is usually preferable to DELETE FROM execution_data; because it is faster and releases the space back immediately. However, note that TRUNCATE is not MVCC-safe, which means concurrent transactions seeing a snapshot taken before the command may not see any rows.
  • Batch Deletion:
    If a straight truncate is not possible due to dependencies or the need to preserve some history, consider deleting in batches (e.g., using a LIMIT clause in a loop) so that the workload is spread out.

Disk Space Reclamation

• Vacuuming Considerations:
  Simply deleting rows (whether by DELETE or batching) may not immediately reduce the disk footprint. If you perform a DELETE, you may eventually need to run a VACUUM FULL to release disk space back to the operating system. Keep in mind that VACUUM FULL rewrites the table and takes an exclusive lock, so plan for maintenance downtime.

• Impact on Performance:
  Actions that rewrite the table (such as TRUNCATE or VACUUM FULL) might lock out concurrent transactions. In a server that’s already under pressure because disk space is nearly full, it might be wise to perform these operations during a maintenance window.

Handling a Nearly Full Disk

Since running out of disk space can lead to issues with Write-Ahead Logging (WAL) and overall server health, taking prompt but careful action is important. Here’s a summary of a safe approach:

1. Verify your backup strategy and ensure a recent backup exists.
2. Confirm that the deletion of all rows in the execution_data table will not violate dependencies or application logic.
3. If it’s safe to delete all rows and no dependent operations require gradual data removal:
   • Use TRUNCATE execution_data; for a rapid, clean deletion.
   • Alternatively, perform batched DELETEs if you need to keep the deletion operation under control or if TRUNCATE’s locking behavior is a concern.
4. After deletion, if you used DELETE (and not TRUNCATE), run a VACUUM FULL execution_data; to reclaim disk space – but schedule this operation during a maintenance window.

Final Thoughts

Deleting (or truncating) all data from a large table is safe provided you’ve addressed backup, dependencies, and operational considerations. This approach can help free up needed space, but if the data turns out to be important later for auditing or reporting, the loss might present challenges. Always test such operations in a staging environment, if possible, before applying them in production.

By following these steps, you can free space while minimizing the risks associated with massive data deletions in PostgreSQL.