ClickHouse Search: Case-sensitive Searches with UPPER & LOWER Functions

Introduction

Implementing a case-sensitive search in ClickHouse, a column-oriented database management system, can be achieved by using the UPPER or LOWER functions. These functions convert text data to either uppercase or lowercase, respectively, allowing for a consistent comparison. Here’s a step-by-step guide on how to do this:

Understanding Case-Sensitivity in ClickHouse

By default, string comparisons in ClickHouse are case-sensitive. However, if you need to enforce case sensitivity in a context where it might not be the default, or if you need to compare strings of different cases, the UPPER and LOWER functions become handy.

Using UPPER and LOWER Functions

The UPPER function converts all characters in a string to uppercase, while LOWER converts them to lowercase. This is useful for performing case-insensitive searches or ensuring case consistency in comparisons.

Practical Example

Suppose you have a table named users with a column username and you want to perform a case-sensitive search for a specific username.

1. Sample Table Creation

CREATE TABLE users (
    id Int32,
    username String
) ENGINE = MergeTree
ORDER BY id;

2. Inserting Sample Data

INSERT INTO users (id, username) VALUES
(1, 'Alice'),
(2, 'alice'),
(3, 'ALICE');

3. Performing Case-Sensitive Search

To find all variations of ‘Alice’, regardless of case:

SELECT * FROM users WHERE LOWER(username) = LOWER('Alice');

Or, if you specifically want to find ‘Alice’ in uppercase:

SELECT * FROM users WHERE UPPER(username) = UPPER('Alice');

Additional Considerations

• Performance: Be aware that using functions like UPPER or LOWER on large datasets can impact query performance, as they need to process each string.
• Character Set: Ensure that the character set and collation of your data are compatible with case transformations.
• Index Usage: Applying functions to a column in a query condition might prevent the use of indexes on that column, potentially slowing down the query.

Conclusion

Using UPPER and LOWER in ClickHouse allows for flexible and case-sensitive or insensitive string comparisons. While they are powerful for ensuring case consistency, always consider the performance implications and test your queries for efficiency, especially when dealing with large datasets.

To learn more about Search in ClickHouse, do consider reading the below articles:

1. ClickHouse Search: Manticore Full Text Search with Realtime Index
2. ClickHouse Search: Manticore Full Text Search with Plain Index
3. Inverted Indexes in ClickHouse for Fast Search: Part 1
4. Inverted Indexes in ClickHouse for Fast Search: Part 2
5. ClickHouse for Vector Search & Storage: Part 1
6. ClickHouse for Vector Search & Storage: Part 2