ClickHouse Troubleshooting: Unraveling Dirty Reads in Index Scans

Data integrity is the compass in the maze of dirty reads. Navigate wisely with ClickHouse. 🧭🔍 #ClickHouse #DataIntegrity


In the world of data, the term “dirty read” sounds like a suspenseful thriller. But in ClickHouse, it’s a real issue that can lead to query confusion and data disorder. Join us as we embark on a journey to uncover the secrets of dirty reads in ClickHouse index scans. We’ll use practical examples and a data matrix to guide us through this intricate maze.

Understanding Dirty Reads in Index Scans

Picture this: while you’re reading a book, someone decides to change the ending without warning. In ClickHouse, a dirty read occurs when a query reads data that’s undergoing changes by another operation, resulting in a perplexing mix of data.

The Enigma of Dirty Reads

Let’s crack the code of common causes behind dirty reads:

Isolation LevelsClickHouse offers isolation levels, but choosing the wrong one can open Pandora’s box of dirty reads.
Concurrency ConundrumWhen multiple queries compete for attention, the stage is set for potential dirty reads.
Index IntrigueDelays in updating indexes can lead to confusion for your queries.
  • A Journey Through Troubleshooting
Choosing IsolationImagine you’re at a bustling market, and you need a quiet place to read. Opt for higher isolation levels like ‘REPLICATED’ or ‘SNAPSHOT’ to ensure your queries aren’t interrupted.
Query OptimizationsThink of queries as your travel itinerary. Streamline them to minimize the time spent wandering through data, reducing the chance of encountering dirty reads.
Index VigilanceJust as you need maps for a successful journey, ClickHouse needs efficient indexes. Ensure they’re updated promptly, and watch out for any delays.
Concurrency ControlImplement traffic lights to control the flow of queries. Locking or versioning mechanisms can prevent collisions between read and write operations.
Query PrioritizationImagine boarding a train before everyone else. Execute write operations before read operations to minimize surprises along the way.

Practical Expedition:

Let’s say you’re running a financial analytics platform on ClickHouse. Traders are firing queries left and right. To protect against dirty reads, employ a ‘SNAPSHOT’ isolation level for those heavy-read queries. This way, everyone sees a consistent snapshot of the data, avoiding unexpected plot twists.


Dirty reads in ClickHouse index scans may resemble a thriller, but they’re not what you want in your data narrative. By deciphering their causes and using our practical solutions, you’ll ensure a smooth data journey where every query reads the right chapter at the right time.

To know more about Indexes in ClickHouse, do read the following articles:

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.