Introduction
In ClickHouse, a mutation refers to any operation that modifies the data in the database, including insert, update and delete operations. These operations can affect the integrity of the data and the accuracy of query results if not used correctly.
- Insert operations add new data to a table.
- Update operations modify existing data in a table.
- Delete operations remove data from a table.
Mutations can cause various issues, such as accidental data loss, stale data, and data inconsistencies. Therefore, it’s important to understand the implications of mutations and take appropriate measures to avoid them or minimize their impact.
ClickHouse supports operations with a high rate of mutations and provides several features to help you manage them. For example, the MergeTree engine, which is recommended for time-series data, has built-in support for deduplication and versioning. This means that you can insert new data without overwriting the existing data, avoiding mutations and preserving the historical data. Additionally, ClickHouse also provides a versioning feature that allows you to keep multiple versions of the same row, that way you can insert new data without overwriting the existing data.
Ways to Avoid Mutations in ClickHouse
Avoiding mutations in ClickHouse is important to maintain the integrity of your data and ensure accurate query results. Here are a few ways to avoid mutations in ClickHouse:
- Use a proper data model: Make sure that you have a well-designed data model that separates your data into appropriate tables and columns. This will help to prevent accidental mutations of your data.
- Use the readonly user: If you need to allow certain users to only read data, you can use the readonly user to avoid accidental mutations. This user can only select data, but cannot update, delete or insert.
- Use the SETTINGS modify_date_column_name query: When you are performing queries that might update the data, you can use the SETTINGS modify_date_column_name query to explicitly set a timestamp column that will be updated with the current time.
- Use the Final keyword: when you are creating tables, you can use the FINAL keyword to prevent any further modifications to the table definition, this will help you to avoid any accidental mutations
- Use the MergeTree engine: MergeTree is the recommended engine when you are dealing with time-series data or when you need to avoid mutations, this engine has built-in support for deduplication and versioning, this means that you can insert new data without overwriting the existing data.
- Use the versioning feature: ClickHouse has a versioning feature that allows you to keep multiple versions of the same row, this means that you can insert new data without overwriting the existing data, this will help you to avoid accidental mutations.
- Use the readonly property: If you want to prevent accidental mutations to a specific table, you can use the readonly property to make the table read-only, this will prevent any updates, deletes, or inserts on that table.
- Use database triggers: you can use database triggers to prevent certain mutations from happening, for example, you can use a trigger to prevent a delete operation on a specific table.
Conclusion
Keep in mind that even with these precautions, you should always backup your data and test your setup in a test environment before applying it to a production environment.
To learn more about Clickhouse updates & troubleshooting, do consider reading the below articles
