Are parallel plans good or bad for ClickHouse Performance?

Shiv Iyer
Posted on May 29, 2023

Parallel query execution in ClickHouse can have both positive and negative effects on performance, depending on the specific use case and the characteristics of the data set. Here’s an explanation of the potential impact of parallel plans on ClickHouse performance using real-life data set examples:

  1. Positive impact of parallel plans: Parallel plans can improve performance in scenarios where the data set is large, and the workload is CPU-bound. By distributing the query workload across multiple threads or nodes, parallel execution can take advantage of the available CPU resources and process the data more quickly. This is particularly beneficial for complex analytical queries involving aggregations, joins, or large-scale data processing.

For example, consider a scenario where you need to perform a complex aggregation query on a large data set of customer transactions. Parallel execution can divide the work among multiple threads or nodes, allowing for faster processing of the data and quicker generation of the results.

  1. Negative impact of parallel plans: While parallel execution can offer performance benefits in certain scenarios, it may have a negative impact in other cases. Parallel plans require additional resources, such as memory and CPU, to execute multiple threads or nodes simultaneously. If the available resources are limited or not properly allocated, parallel execution can lead to resource contention and performance degradation.

In situations where the data set is relatively small or the workload is I/O-bound rather than CPU-bound, parallel plans may not provide significant performance gains. Additionally, parallel execution can introduce additional overhead for thread coordination and synchronization, which can impact performance for queries that have relatively low computational complexity.

For example, suppose you have a small lookup table that needs to be joined with a larger fact table in a simple query. In this case, parallel execution may not significantly improve performance and can potentially introduce overhead due to thread coordination.

It’s important to carefully analyze your specific data set, query patterns, and available resources to determine whether parallel plans are suitable for your ClickHouse deployment. Benchmarking and performance testing with real-life data sets and workloads are crucial to understanding the impact of parallel execution on performance. Adjusting configuration parameters, such as max_threads or max_distributed_connections, can also help optimize parallel execution behavior based on your hardware and workload characteristics.