Non-SARGable (Search ARGument ABLE) predicates are conditions in SQL queries that prevent the database engine from using indexes efficiently, leading to full table scans and degraded query performance. Implementing and handling Non-SARGable predicates in ClickHouse involves understanding how these predicates affect query performance and how to optimize them for better efficiency. Below is an explanation of how Non-SARGable predicates are implemented and optimized in ClickHouse:
Understanding Non-SARGable Predicates
Non-SARGable predicates are those that involve operations on columns in a way that precludes the use of indexes. Common examples include:
• Functions applied to columns (e.g., LOWER(column) = ‘value’)
• Arithmetic operations on columns (e.g., column + 1 = 10)
• Type conversions (e.g., CAST(column AS VARCHAR) = ‘value’)
Implementation in ClickHouse
ClickHouse, being a columnar database, processes and optimizes queries differently compared to traditional row-based databases. Here’s how Non-SARGable predicates might be handled:
1. Full Table Scans: If a query contains Non-SARGable predicates, ClickHouse may perform a full table scan instead of using indexes. This is because the operation on the column value prevents the direct use of any existing index.
2. Function Evaluation: ClickHouse evaluates functions applied to columns at runtime, which can slow down query performance significantly if these functions are applied to a large number of rows.
Optimizing Non-SARGable Predicates
To optimize queries with Non-SARGable predicates in ClickHouse, consider the following strategies:
1. Rewrite Queries: Whenever possible, rewrite the query to make the predicate SARGable. For example:
• Instead of LOWER(column) = ‘value’, store the values in lowercase and use column = ‘value’.
• Instead of column + 1 = 10, rewrite it as column = 9.
2. Pre-compute Values: Pre-compute values that would otherwise be calculated at query time. This can be done using materialized views or adding additional columns to store pre-computed values.
3. Use Appropriate Indexes: ClickHouse supports various types of indexes (e.g., primary keys, secondary indexes). Ensure that indexes are created on columns that are frequently used in WHERE clauses.
4. Optimize Data Layout: Ensure that the data is organized and partitioned in a way that minimizes the need for full table scans. For example, use partitioning on columns that are commonly used in filtering.
Example
Let’s consider an example where a query contains a Non-SARGable predicate:
SELECT * FROM my_table WHERE TOLOWERCASE(column_name) = 'value';
This query prevents ClickHouse from using indexes effectively. To optimize it, you can rewrite the query and adjust the data storage:
1. Store Lowercase Values: Store column_name values in lowercase:
ALTER TABLE my_table UPDATE column_name = lower(column_name);
2. Rewrite Query: Rewrite the query to remove the function:
SELECT * FROM my_table WHERE column_name = 'value';
By implementing these optimizations, the query becomes SARGable, allowing ClickHouse to utilize indexes more effectively and improving query performance.
Conclusion
Non-SARGable predicates can significantly impact query performance in ClickHouse by preventing the efficient use of indexes and causing full table scans. By rewriting queries to make predicates SARGable, pre-computing values, using appropriate indexes, and optimizing data layout, you can mitigate the performance impact of Non-SARGable predicates and ensure more efficient query execution in ClickHouse.