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
- Regularly analyze the
query_log
to identify slow-running or resource-intensive queries. - Use the
query_log
in conjunction with other system tables likesystem.processes
for comprehensive performance monitoring. - Implement a retention policy for the
query_log
to manage its size and prevent excessive disk usage. - Utilize the
query_log
for auditing purposes and tracking user activity. - Consider setting up alerts based on specific metrics in the
query_log
, such as query duration exceeding a threshold.
Common Issues or Misuses
- Overlooking the
query_log
size, which can grow rapidly in high-traffic environments. - Failing to secure access to the
query_log
, as it may contain sensitive information. - Not considering the performance impact of excessive logging, especially for high-frequency queries.
- Misinterpreting query times without considering factors like data volume or concurrent queries.
- 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 startedquery_duration_ms
: Query execution time in millisecondsread_rows
: Number of rows read during query executionmemory_usage
: Memory consumed by the queryexception
: 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.