join_use_nulls is a setting in ClickHouse that determines how NULL values are handled during JOIN operations. When enabled (set to 1), it causes non-matched rows to be filled with NULL values instead of using default values. This setting affects the behavior of LEFT, RIGHT, and FULL OUTER JOINs, influencing both the result set and query performance.
Best practices
- Enable
join_use_nullswhen working with data that may contain NULL values or when consistency with standard SQL behavior is required. - Use
join_use_nulls=1when performing complex analytical queries that rely on the presence of NULL values for accurate results. - Consider the impact on query performance when enabling this setting, as it may introduce additional overhead for NULL checks.
- Test queries with both enabled and disabled
join_use_nullsto understand the impact on your specific use case and data set. - Document the use of this setting in your project to ensure consistency across different queries and applications.
Common issues or misuses
- Inconsistent results: Forgetting to set
join_use_nullsconsistently across different queries or applications can lead to unexpected and inconsistent results. - Performance impact: Enabling
join_use_nullsmay introduce a slight performance overhead, especially for large-scale joins. - Misinterpreting results: Users unfamiliar with this setting may misinterpret query results when
join_use_nullsis disabled, as non-matched rows will contain default values instead of NULLs. - Compatibility issues: Code or queries designed for standard SQL databases may behave differently in ClickHouse if
join_use_nullsis not properly configured.
Additional information
The join_use_nulls setting can be configured at different levels:
- Server-wide in the configuration file
- Per-session using SET command
- Per-query by specifying it in the SETTINGS clause
When join_use_nulls is disabled (set to 0), ClickHouse uses default values for non-matched rows:
- Numeric types: 0
- Strings: Empty string
- Date and DateTime: Earliest date (1970-01-01) or zero unix timestamp
Frequently Asked Questions
Q: How does join_use_nulls affect query performance?
A: Enabling join_use_nulls may introduce a slight performance overhead due to additional NULL checks during join operations. However, the impact is usually minimal and outweighed by the benefits of consistent NULL handling in many analytical scenarios.
Q: Can join_use_nulls be used with all types of JOINs in ClickHouse?
A: join_use_nulls primarily affects LEFT, RIGHT, and FULL OUTER JOINs. It has no impact on INNER JOINs as they only return matched rows.
Q: How do I enable join_use_nulls for a specific query?
A: You can enable join_use_nulls for a specific query by adding it to the SETTINGS clause. For example: SELECT ... FROM ... JOIN ... SETTINGS join_use_nulls=1
Q: Does join_use_nulls affect the storage of data in ClickHouse?
A: No, join_use_nulls only affects the behavior of JOIN operations during query execution. It does not change how data is stored in ClickHouse tables.
Q: Is join_use_nulls enabled by default in ClickHouse?
A: By default, join_use_nulls is disabled (set to 0) in ClickHouse. You need to explicitly enable it either in the server configuration, session settings, or per-query basis if you want NULL values for non-matched rows in JOIN operations.
