Partial indexes are a powerful feature in ClickHouse that allow DBAs to index only a subset of the rows in a table based on a specified condition. This can significantly reduce the index size and improve the query performance for specific queries.
To implement partial indexes in ClickHouse, the following steps can be followed:
- Define a new index with the PARTITION BY clause to specify the partitioning key, and the WHERE clause to specify the condition for the partial index. For example, to create a partial index on the orders table for orders with a total greater than 100, the following query can be used:
CREATE INDEX orders_partial_idx ON orders (cust_id, order_date)
PARTITION BY toYYYYMM(order_date)
WHERE total > 100
2. Run the OPTIMIZE INDEX command to build the index. This command will scan the table and create the partial index on the subset of the rows that meet the specified condition. For example:
OPTIMIZE TABLE orders FINAL
Once the partial index has been created, queries that match the WHERE clause will benefit from the improved query performance, while queries that do not match the WHERE clause will not use the index.
The benefits of using partial indexes in ClickHouse are as follows:
- Reduced index size: Partial indexes only index a subset of the rows in a table, which can significantly reduce the index size and improve the query performance.
- Improved query performance: Partial indexes can improve the query performance for queries that match the specified condition by reducing the number of rows that need to be scanned.
- Flexibility: Partial indexes provide flexibility to index specific subsets of the data based on custom criteria, allowing DBAs to optimize their database for specific query patterns.
- Reduced maintenance overhead: Partial indexes require less maintenance compared to full indexes, as they only need to be updated when the subset of data they index changes.
In summary, partial indexes are a powerful feature in ClickHouse that can significantly improve query performance for specific query patterns. By implementing partial indexes, DBAs can optimize their database for specific workloads, reduce index size, and improve query performance.