When to Avoid Indexing in ClickHouse for Optimal Performance

Introduction

Indexing is a powerful technique that enhances database query performance by allowing rapid data retrieval. However, there are scenarios where indexing might not be the optimal choice. This article delves into situations where avoiding indexing in ClickHouse could lead to better performance. Real-life data sets and use cases will illustrate when indexing might negatively impact performance and provide insights into best practices for indexing in ClickHouse.

When to Avoid Indexing

1. Low Cardinality Columns: Indexing might not be beneficial if a column has very low cardinality (few distinct values). In ClickHouse, index lookup can sometimes be more resource-intensive than scanning the entire data for such columns.

2. Frequent Data Modifications: Indexes require maintenance during data modifications like inserts, updates, and deletes. In cases where data is frequently modified, maintaining indexes can become a performance overhead, slowing down these operations.

3. Small Tables: The overhead of index maintenance might outweigh the benefits for small tables that can fit into memory entirely. Scanning the entire table could be faster than index lookups.

Negative Impact of Indexing

1. Overhead on Write Operations: Indexes introduce overhead during write operations. Maintaining indexes while performing frequent inserts, updates, and deletes can slow down write-intensive workloads.

2. Increased Storage Requirements: Indexes consume additional storage space. In cases where disk space is a concern, unnecessary indexes can lead to inefficient utilization.

Best Practices for Indexing:

1. Selectively Index: Index only columns that are frequently used in query predicates or join conditions. Indexing every column can lead to unnecessary overhead.

2. Consider Memory: ClickHouse’s data is heavily memory-oriented. Indexes can consume memory, so consider the available memory and the impact on overall performance.

3. Avoid Over-Indexing: Evaluate the trade-off between index creation and query performance. Over-indexing might lead to diminishing returns, especially for low-cardinality columns.

Use Case

Imagine a financial institution maintaining a transaction log. The “transaction_type” column in this log might have only a few distinct values (e.g., “purchase,” “withdrawal”). Creating an index on this column could be counterproductive as the index might be almost as large as the table itself, and query performance might not significantly improve.

On the other hand, a column like “transaction_date” with high cardinality would benefit from indexing, as it’s often used in queries for time-based analysis.

Conclusion

Indexing is a double-edged sword. While it significantly improves query performance in many cases, it’s crucial to be cautious and consider the specific characteristics of your data and workload. Avoid indexing low-cardinality columns, be mindful of frequent data modifications, and always evaluate the trade-off between indexing and query performance. In ClickHouse, indexing can be a powerful ally, but its application should be tailored to your unique dataset and usage patterns to maximize performance gains.

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About Shiv Iyer 216 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.