allow_suspicious_low_cardinality_types is a setting in ClickHouse that controls whether the system allows the use of LowCardinality for data types that typically have high cardinality. When enabled, it permits the creation of LowCardinality columns for types like Float32, Float64, or types with a large range of possible values, which might not benefit from this optimization and could potentially harm performance.
Best Practices
- Keep
allow_suspicious_low_cardinality_typesdisabled by default. - Only enable it when you have a specific use case that requires LowCardinality for high-cardinality types.
- Conduct thorough performance testing before and after applying LowCardinality to suspicious types.
- Monitor query performance and storage efficiency when using LowCardinality on atypical data types.
- Regularly analyze the cardinality of your data to ensure LowCardinality remains beneficial.
Common Issues or Misuses
- Applying LowCardinality to high-cardinality columns, leading to decreased performance.
- Enabling
allow_suspicious_low_cardinality_typeswithout understanding its implications. - Overusing LowCardinality on inappropriate data types, resulting in increased memory usage.
- Failing to reassess the effectiveness of LowCardinality as data characteristics change over time.
- Neglecting to benchmark performance before and after applying LowCardinality to suspicious types.
Additional Information
The LowCardinality data type in ClickHouse is designed to optimize storage and query performance for columns with a limited number of distinct values. It works by storing a dictionary of unique values and using integer keys to reference these values. While this can significantly improve performance for truly low-cardinality data, applying it to high-cardinality types can have the opposite effect.
The allow_suspicious_low_cardinality_types setting is a safeguard to prevent accidental misuse of LowCardinality. By default, ClickHouse restricts its application to types that are likely to benefit from this optimization. However, there may be edge cases where using LowCardinality on typically high-cardinality types could be advantageous, which is why this setting exists.
Frequently Asked Questions
Q: When should I consider enabling allow_suspicious_low_cardinality_types?
A: You should only enable this setting if you have a specific use case where you've determined through testing that applying LowCardinality to typically high-cardinality types (like Float32 or Float64) provides a performance benefit. Always conduct thorough benchmarking before and after enabling this setting.
Q: What are the risks of enabling allow_suspicious_low_cardinality_types?
A: The main risks include potential performance degradation, increased memory usage, and longer insert times if LowCardinality is applied to truly high-cardinality columns. It may also lead to unexpected behavior in queries and data processing if not carefully managed.
Q: How can I determine if a column is suitable for LowCardinality?
A: Analyze the number of distinct values in the column relative to the total number of rows. If the ratio of unique values to total rows is low (typically less than 1%), it might be a good candidate for LowCardinality. Also, consider the data distribution and query patterns that access this column.
Q: Can I change a column from regular type to LowCardinality after data has been inserted?
A: Yes, you can alter the table structure to change a column to LowCardinality. However, this operation can be resource-intensive for large tables. It's best to plan your schema with LowCardinality in mind from the beginning if possible.
Q: How does allow_suspicious_low_cardinality_types affect query performance?
A: If used inappropriately, it can negatively impact query performance by increasing the overhead of encoding and decoding values for high-cardinality columns. However, in some edge cases where the data distribution is unexpectedly favorable, it might improve performance. Always measure the impact on your specific workload.
