Troubleshooting ClickHouse Data Skew in Distributed Aggregation

Troubleshooting data skew in distributed aggregations in ClickHouse, particularly in a sharded setup, is a complex but critical task to ensure balanced workload distribution and optimal query performance. Here’s a structured approach to diagnosing and addressing data skew:

1. Identify the Data Skew

  • Monitoring and Metrics: Use monitoring tools to observe the workload distribution across the shards. Look for discrepancies in CPU, memory, and I/O usage.
  • Query Performance Analysis: Examine the execution time and resource utilization of aggregation queries. Significantly longer execution times for specific shards indicate skew.
  • EXPLAIN Queries: Use EXPLAIN to understand how ClickHouse executes the aggregation queries. Look for steps where data is unevenly processed across shards.

2. Analyze the Data Distribution

  • Check Sharding Key: Review the sharding key used in the distributed table. An effective sharding key should distribute data evenly across all shards.
SELECT sharding_key, COUNT(*) FROM distributed_table GROUP BY sharding_key;
  • Data Cardinality: Investigate the cardinality of the columns involved in aggregation. Low cardinality or uneven distribution can lead to skew.

3. Examine Query Design

  • Aggregation Keys: Check if the aggregation keys or conditions are leading to data skew. Some keys might have significantly more data associated with them.
  • Suboptimal Joins: Determine if joins in your queries contribute to the skew, especially if they are not co-located.

4. Addressing Data Skew

  • Modify Sharding Key: If the sharding key is the culprit, consider changing it to a key with a more uniform distribution.
  • Pre-aggregate Data: Use materialized views to pre-aggregate data. This can help in reducing the amount of data shuffled during query execution.
  • Optimize Queries: Rewrite queries to minimize skew. For instance, filtering data before aggregation can reduce the load on specific shards.
  • Rebalance Data: In some cases, manually redistributing data or adding more shards might be necessary.

5. Validate Changes

  • Testing: After making changes, test the queries again to ensure that the data skew is resolved or significantly reduced.
  • Continuous Monitoring: Keep monitoring the system to catch any future occurrences of data skew.

6. Best Practices

  • Even Data Distribution: Ensure that new data inserted into the cluster maintains an even distribution.
  • Regular Review: Periodically review the sharding strategy and query patterns to avoid data skew.
  • Load Testing: Regularly perform load testing to simulate real-world data scenarios and identify potential skew issues.


Resolving data skew in distributed aggregations in ClickHouse requires a combination of careful analysis of data distribution, sharding strategies, and query patterns. It’s often an iterative process involving adjustments to the database schema, query design, and possibly the sharding setup. Continuous monitoring and periodic reviews are essential to maintain optimal performance in a distributed ClickHouse environment.