Does Column Order in MySQL Multi-Column Indexes Affect Query Performance?

Field order in MySQL multi-column index: does it matter?

Question:

When creating a multi-column index in MySQL, will the order of specifying columns affect performance? For example, does using INDEX name (last_name, first_name) produce different results than using INDEX name (first_name, last_name)?

Answer:

When thinking about a multi-column index, you can compare it to a phone book. A phone book is essentially an index sorted by last name, then first name. This determines the search order, with the initial "column" providing the main sorting criterion.

Queries are divided into several categories:

• Exact value lookup: For searches such as "Find all Smiths", the surname index can be of immediate help since the entries are arranged alphabetically.
• Partial value lookups: Searching like "find all John" is not efficient because Johns are scattered throughout the directory.
• Combined value lookups: For searches of a specific combination, such as "Find everyone with the last name Smith and the first name John", an index sorted by last name and then by first name can be helpful.

If the phone book were sorted by first name, then last name, then it would facilitate queries for cases 2 and 3, but not case 1.

In addition to exact value lookups, consider range queries:

• For example, to find all Johns whose last names begin with "S", if the phone book uses a sort order of last name then first name, you would need to scan the entire directory. In contrast, an index sorted by first name and then last name would group all Johns together, making range queries more efficient.

Conclusion:

The order of columns in a multi-column index is critical, as different types of queries may require specific column ordering for optimal performance. Complex queries may require multiple indexes with different column orders to accommodate various use cases.

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