ClickHouse Query Log: Tracking and Analyzing Query Performance

The query_log in ClickHouse is a system table that records information about executed queries. It provides detailed insights into query performance, execution times, resource usage, and other relevant metrics. This log is an essential tool for database administrators and developers to monitor, analyze, and optimize query performance in ClickHouse.

Best Practices

1. Regularly analyze the query_log to identify slow-running or resource-intensive queries.
2. Use the query_log in conjunction with other system tables like system.processes for comprehensive performance monitoring.
3. Implement a retention policy for the query_log to manage its size and prevent excessive disk usage.
4. Utilize the query_log for auditing purposes and tracking user activity.
5. Consider setting up alerts based on specific metrics in the query_log, such as query duration exceeding a threshold.

Common Issues or Misuses

1. Overlooking the query_log size, which can grow rapidly in high-traffic environments.
2. Failing to secure access to the query_log, as it may contain sensitive information.
3. Not considering the performance impact of excessive logging, especially for high-frequency queries.
4. Misinterpreting query times without considering factors like data volume or concurrent queries.
5. Neglecting to correlate query_log data with other system metrics for a holistic view of performance.

Additional Information

The query_log table structure includes columns such as:

• query_start_time: When the query execution started
• query_duration_ms: Query execution time in milliseconds
• read_rows: Number of rows read during query execution
• memory_usage: Memory consumed by the query
• exception: Any exception that occurred during query execution

To access the query_log, you can use SQL queries like:

SELECT * FROM system.query_log WHERE event_date = today() ORDER BY query_start_time DESC LIMIT 10

Frequently Asked Questions

Q: How can I enable or disable the query_log in ClickHouse?
A: The query_log is enabled by default. You can disable it by setting log_queries=0 in the ClickHouse configuration file. To re-enable it, set log_queries=1.

Q: What's the difference between query_log and query_thread_log?
A: While query_log provides information about entire queries, query_thread_log offers more granular data about individual threads used in query execution, useful for analyzing parallel query performance.

Q: How long are records kept in the query_log?
A: The retention period for query_log entries is configurable. By default, it's set to 30 days, but you can adjust this using the query_log_ttl setting in the ClickHouse configuration.

Q: Can I use the query_log to identify problematic queries in real-time?
A: Yes, you can query the query_log in real-time to identify long-running or resource-intensive queries. However, for immediate action on currently running queries, system.processes might be more suitable.

Q: Is it possible to exclude certain queries from being logged in query_log?
A: Yes, you can use the log_queries_probability setting to control the probability of logging queries. Setting it to a value less than 1 will cause only a fraction of queries to be logged, which can be useful for reducing the log size in high-traffic scenarios.