How Does Column Order in MySQL Multi-Column Indexes Impact Query Performance?

The importance of field order in MySQL multi-column index

Multi-column indexes enhance the performance of database operations. However, one aspect that is often overlooked is the impact of the order in which columns are specified in these indexes. Understanding the role of field order is critical to optimizing index use.

Multi-column indexing and search performance

Similar to a phone book, a multi-column index acts as an ordered directory of database records. The order of columns in an index is similar to the sort order of a directory. For example, an index on (last_name, first_name) organizes records first by last name and then by first name.

Use field order to refine searches

The order of columns in a multi-column index affects search performance depending on the nature of the query:

• Exact value search: If the query searches for the exact value of two columns, the order of the columns in the index does not matter.
• Range Search: For range queries (searching for values ​​falling within a specific range), the order of columns plays a crucial role. An index with column order (first_name, last_name) will effectively locate records based on first name, and within the group based on last name. In contrast, if the column order is (last_name, first_name), then a name-based range query requires a full scan of the index.

Specific case studies

Consider the following query:

SELECT *
FROM test
WHERE last_name LIKE 'S%' AND first_name = 'John';

• Using index (last_name, first_name), records will be sorted by last name first, making searches for last_name LIKE 'S%' more efficient. However, the records are not grouped by name, which makes searching for first_name = 'John' inefficient.
• Instead, using index (first_name, last_name), the records will be grouped by name, making it possible to efficiently search for first_name = 'John'. However, searching for last_name LIKE 'S%' is inefficient since grouping by last name is not possible.

Therefore, the order of columns in this multi-column index affects the search performance of range queries based on last_name and first_name.

Conclusion

The order of columns in a multi-column index in MySQL has a significant impact on search performance, especially for range queries. Understanding the impact of field order allows database administrators to optimize index usage for specific types of queries, thereby improving application performance.

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