How can NULL values affect ClickHouse Performance?

Shiv Iyer
Posted on February 25, 2023

NULL values in ClickHouse can affect performance in several ways. One way is that NULL values can increase the size of data, which can lead to slower query execution times as more data needs to be scanned. Additionally, NULL values can also make it difficult to use indexes effectively, as the index will not contain any entries for NULL values. This can lead to more disk I/O, as the database needs to scan more data to find the relevant rows. Furthermore, NULL values can also lead to unexpected results when performing calculations or aggregations, as NULL values are not included in these operations. To avoid these issues, it is important to understand how NULL values are used in your data and to make sure that they are handled correctly in your queries.

Troubleshooting ClickHouse Performance with NULL values

To troubleshoot ClickHouse performance issues related to NULL values, you can follow these steps:

  1. Identify the tables and columns that have a high percentage of NULL values. You can use the following SQL query to check the NULL percentage for each column in a table:
SELECT column, 100.0 * null_values / total_values AS percentage_null 
FROM system.columns 
WHERE database = 'your_database' AND table = 'your_table' 
ORDER BY percentage_null DESC;
  1. Check the indexes on the identified columns. If the columns with a high percentage of NULL values are not indexed, adding an index can improve query performance.
  2. Analyze the queries that are causing performance issues. If the queries are using the columns with a high percentage of NULL values as filters, you can try to rewrite the queries to use other columns or indexes.
  3. Check the compression settings for the columns with a high percentage of NULL values. If the compression settings are not optimal, you can try adjusting them to reduce the size of the data on disk and improve query performance.
  4. Monitor the performance of the queries and the system as a whole. Use ClickHouse’s built-in performance monitoring tools, such as system.events, to track query execution time, memory usage, and disk I/O.

It is important to note that NULL values do not have any performance implications in ClickHouse as the engine is designed to work with nullable columns and missing values efficiently.