log_queries is a ClickHouse setting that enables the logging of executed queries. When enabled, ClickHouse records information about each query, including its text, execution time, and various performance metrics. This feature is crucial for monitoring, debugging, and optimizing database performance.
Best Practices
- Enable
log_queriesin production environments for performance monitoring and troubleshooting. - Use
system.query_logtable to analyze logged queries. - Implement log rotation to manage log file sizes.
- Set appropriate log levels to balance between information detail and system performance.
- Regularly review and analyze query logs to identify optimization opportunities.
Common Issues or Misuses
- Excessive logging leading to increased disk usage and potential performance impact.
- Failing to secure query logs, which may contain sensitive information.
- Not setting up log rotation, resulting in large, unmanageable log files.
- Overlooking the performance impact of logging very frequent or complex queries.
- Misinterpreting query execution times without considering concurrent system load.
Additional Information
The log_queries setting works in conjunction with the system.query_log table, which stores the logged information. Administrators can query this table to gain insights into query patterns, performance bottlenecks, and user activity.
To enable query logging, set:
SET log_queries=1;
You can also configure more granular settings like log_query_threads and log_query_settings for additional details.
Frequently Asked Questions
Q: How do I enable query logging in ClickHouse?
A: You can enable query logging by setting log_queries=1 in the ClickHouse configuration file or using the SET log_queries=1; SQL command for the current session.
Q: Where are the query logs stored in ClickHouse?
A: Query logs are stored in the system.query_log table. You can query this table to access the logged information.
Q: Does enabling log_queries affect ClickHouse performance?
A: While logging does introduce some overhead, the impact is generally minimal. However, for high-load systems with frequent queries, it's important to monitor and adjust logging settings as needed.
Q: How long are query logs retained in ClickHouse?
A: The retention period for query logs can be configured using the query_log_ttl setting. By default, logs are kept for 30 days.
Q: Can I log only slow queries in ClickHouse?
A: Yes, you can use the log_queries_cut_to_length setting to log only queries that exceed a certain execution time, helping to focus on potentially problematic queries.
