How to diagnose and fix the 72000 snapshot_too_old error code in Postgres. 

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The 72000: snapshot_too_old error code in PostgreSQL is not a standard PostgreSQL error and seems to be a confusion with the Oracle error ORA-01555: snapshot too old. PostgreSQL does not use the same error code system as Oracle, and while it has a concept of snapshots for transaction isolation, the specific “snapshot too old” error is traditionally associated with Oracle databases.

In Oracle, the ORA-01555: snapshot too old error occurs when a query attempts to read data from a consistent view (snapshot) of the database, but the undo information needed to provide this view has been overwritten. This can happen in long-running queries if the undo retention time is too short or the undo tablespace is too small to hold all the necessary undo records.

In PostgreSQL, while you won’t encounter an ORA-01555 error, you might face similar issues related to old snapshots, especially when using long transactions or with specific configurations that involve cleaning up old snapshots, like the old_snapshot_threshold setting. This setting in PostgreSQL controls the minimum amount of time a query snapshot can be used without the risk of encountering a “snapshot too old” error (PostgreSQL Documentation).

If you’re running into snapshot-related issues in PostgreSQL, here are some general steps to diagnose and address the problem:

  1. Check Long-Running Queries:
    Ensure that you do not have long-running queries that might be holding onto snapshots for an extended period. You can identify long-running queries using the pg_stat_activity view.
   SELECT pid, now() - pg_stat_activity.query_start AS duration, query, state
   FROM pg_stat_activity
   WHERE (now() - pg_stat_activity.query_start) > interval '5 minutes';
  1. Review old_snapshot_threshold:
    If you have set the old_snapshot_threshold parameter, review its value to ensure it’s not causing snapshots to be considered too old prematurely. The default setting of -1 disables the feature, meaning snapshots are not marked as old based on time.
  2. Vacuum and Autovacuum:
    Make sure that the vacuum (and autovacuum) processes are running appropriately to prevent transaction ID wraparound and to clean up dead tuples, which can affect snapshot visibility.
   VACUUM (VERBOSE, ANALYZE);
  1. Monitoring and Configuration:
    Monitor your database for transaction wraparound using pg_stat_database and adjust your autovacuum settings to ensure it runs more frequently if necessary.
  2. Transaction Management:
    Review your application’s transaction management strategy. Long transactions can hold snapshots for extended periods, which can lead to issues. Try to keep transactions as short as possible.
  3. Increase Resources:
    If you’re encountering issues related to insufficient resources (similar to Oracle’s undo tablespace), consider increasing the resources allocated to PostgreSQL, such as work memory (work_mem) and maintenance work memory (maintenance_work_mem).

Remember, the exact steps to diagnose and fix issues related to old snapshots will depend on the specific circumstances and configuration of your PostgreSQL server. Always ensure you have proper backups before making significant changes to your database configuration.

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