Optimizing High-Velocity, High-Volume ETL Operations with Data Skipping Indexes in ClickHouse

Data Skipping Indexes in ClickHouse are an effective optimization tool for enhancing query performance in high-velocity, high-volume ETL operations. These indexes help by allowing the database to skip over blocks of data that do not match the query conditions, significantly reducing the amount of data read from the disk during query execution. Here’s how they contribute to handling large-scale ETL processes efficiently:

Concept and Functionality

  • Selective Data Reads: Data Skipping Indexes maintain metadata about the ranges of values in each data block or partition. When a query is executed, ClickHouse can quickly check this metadata to determine whether a block contains relevant data. If the block does not meet the query criteria, it is skipped entirely, avoiding unnecessary data reads.
  • Index Types: ClickHouse supports various types of data skipping indexes, such as min/max, set, bloom filter, and ngram. Each type is designed to optimize different kinds of queries, such as range queries, membership checks, or pattern matching.

Impact on High-Velocity, High-Volume ETL

  • Efficient Data Ingestion: During the ETL process, data is often ingested at a rapid pace into ClickHouse. Data skipping indexes reduce the overhead during the transformation and loading phases by minimizing the volume of data processed during query-driven transformations.
  • Query Optimization: ETL operations frequently involve complex queries that aggregate and transform data. Data skipping indexes improve the speed of these operations by focusing on relevant data, thereby speeding up aggregation and filtering tasks.

Practical Applications

  • Time-Series Data: In ETL processes involving time-series data, such as logs or financial transactions, data skipping indexes can be configured to skip irrelevant time periods quickly. For example, a min/max index on a timestamp column can bypass blocks outside a specific time range.
  • High-Cardinality Fields: For fields with high cardinality, such as user IDs or product codes, set or bloom filter indexes can efficiently exclude blocks that do not contain certain values, which is particularly useful when processing large batches of data.

Configuration Tips

  • Choosing the Right Index: Select the type of data skipping index based on the nature of the queries most commonly executed. For instance, if your ETL process involves checking the presence of specific values, a bloom filter index might be more appropriate.
  • Balancing Cost and Performance: While data skipping indexes can drastically improve query performance, they also require additional storage and maintenance. Evaluate the trade-offs between faster query performance and increased resource usage.


Data Skipping Indexes in ClickHouse provide a crucial performance optimization for high-velocity, high-volume ETL operations. By enabling more efficient query processing through selective data reads, these indexes help maintain high performance and scalability, even as data volumes and query complexity grow. Properly implemented, they ensure that ClickHouse remains a robust solution for large-scale data analytics environments.

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About Shiv Iyer 222 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.