Learn how to deal with Hot Standby replicas and server-side cursor issues. Our PostgreSQL Support Team is here to help.

How to deal with Hot Standby replicas and Server-side Cursor issue

In PostgreSQL and other database management systems, two commonly encountered concepts are Hot Standby replicas and server-side cursors. Although both features are extremely useful, they can also introduce complications in certain scenarios.

Today, we will break down what these features are, the causes of issues, and how to resolve them.

For general PostgreSQL troubleshooting, users often face common errors such as the “relation does not exist” error, which usually points to query or schema issues. By comparison, hot standby and cursor errors are more about replication and query execution states.

Understanding Hot Standby Replicas

Hot Standby replication is a PostgreSQL feature that allows the creation of a read-only replica of the primary database. This enables read queries on the replica while the primary continues handling write operations.

Hot standby mode is handy for:

• Keeping standby servers in sync with the primary.
• Restoring a backup to a precise state while still allowing queries.
• Letting users run queries while the server transitions from recovery mode to normal operation.

Running queries in hot standby mode is similar to working on the primary database, but there are administrative differences, especially when dealing with long-running queries. In some cases, issues may feel similar to other errors, like the PostgreSQL “fatal: database root does not exist” error, where configuration inconsistencies disrupt database accessibility.

Understanding Server-Side Cursors

Server-side cursors allow applications to fetch large result sets incrementally instead of loading everything into memory at once. The database maintains cursor state, and the client retrieves rows as needed.

While this helps manage memory and performance, issues can arise when replicas are queried with server-side cursors. Since these cursors depend on the primary server state, replication lag or conflicts can disrupt query execution.

Causes of Hot Standby and Cursor Issues

• Hot Standby Replicas:

  Replicas falling behind the primary due to:

  • Network delays.
  • Heavy query load.
  • Slow replication performance.
• Server-Side Cursors
  • Long-running queries that rely on row versions are no longer available.
  • Cursors needing state management on the primary while queries execute on the standby.

Common Replication Conflicts

Replication conflicts occur when standby queries and primary operations clash. For example:

• The primary removes old row versions that a long-running standby query still needs.
• Queries on the standby conflict with exclusive locks on the primary. Examples include `ALTER TABLE`, `DROP TABLE`, `TRUNCATE`, `CREATE INDEX`, `CLUSTER`, and even VACUUM truncations.

If not handled properly, such conflicts can cause interruptions similar to installation-level issues, like the PostgreSQL readline library not found error, which arises when required dependencies are missing.

Solutions: 

If queries fail on the standby due to replication conflicts, we have two main options:

1. Enable hot\_standby\_feedback

   ALTER SYSTEM SET hot_standby_feedback = on;

   This prevents the primary from removing row versions needed by standby queries. This helps long-running standby queries succeed.
2. Adjust max\_standby\_streaming\_delay

   ALTER SYSTEM SET max_standby_streaming_delay = '10min';  -- or -1 for unlimited

   This delays applying conflicting changes from the primary until standby queries complete. It gives standby queries more time to finish.

Both solutions involve trade-offs. The right choice depends on workload characteristics and tolerance for lag or bloat.

General Solutions

• For Hot Standby Replicas
  • Ensure replication is properly tuned and monitored.
  • Use dedicated monitoring tools to detect lag early.
  • Design applications with the read-only nature of replicas in mind.
• For Server-Side Cursors
  • Close cursors properly after use.
  • Avoid excessively long-running queries on replicas.
  • Optimize query design to minimize resource consumption.
  • Consider pagination or batch processing for large result sets.

For instance, if working with complex data types such as hstore, query optimization and updates must be carefully managed. See this guide on PostgreSQL updating hstore fields for practical tuning insights.

[Need assistance with a different issue? Our team is available 24/7.]

Conclusion

Both Hot Standby replicas and server-side cursors are valuable PostgreSQL features, but they require careful configuration and monitoring to avoid problems.

For deeper troubleshooting, you may also want to review common PostgreSQL errors like the relation does not exist, database root does not exist, or readline library not found errors, which often appear in real-world deployments.

In brief, our Support Experts demonstrated how to deal with Hot Standby replicas and server-side cursor issues.