MySQL Index Cardinality Indexes

Indexes in MySQL are data structures that improve the speed of data retrieval operations on a database table by providing quick access paths to data. Index cardinality refers to the uniqueness of values in an index, with higher cardinality indicating a larger number of unique values.

Usage

Indexes are used to enhance database performance by allowing MySQL to locate and access data efficiently, especially in large tables. They are typically employed when executing SELECT queries, sorting data, or enforcing uniqueness.

CREATE INDEX index_name
ON table_name (column_name);

In this syntax, CREATE INDEX creates an index named index_name on the specified column of table_name, improving query performance on that column. While indexes speed up data retrieval, they can slow down data modification operations like INSERT, UPDATE, and DELETE due to the overhead of maintaining the index.

Examples

1. Creating a Simple Index

CREATE INDEX idx_last_name
ON employees (last_name);

This example creates an index on the last_name column of the employees table, which can speed up queries filtering or sorting by last name.

2. Creating a Composite Index

CREATE INDEX idx_full_name
ON employees (first_name, last_name);

Here, a composite index is created on both first_name and last_name, optimizing searches that involve both columns together.

3. Checking Index Cardinality

SHOW INDEX
FROM employees
WHERE Key_name = 'idx_last_name';

This command retrieves details about the idx_last_name index, including its cardinality, which helps understand the number of distinct values and assess performance impact. Higher cardinality typically improves performance for equality queries but may not be as beneficial for range queries.

Tips and Best Practices

• Use indexes judiciously. While indexes can speed up data retrieval, they can also slow down data insertion, deletion, and updates.
• Prioritize high-cardinality columns. Index columns with high cardinality for maximum performance improvement, especially for equality searches.
• Avoid excessive indexing. Too many indexes can lead to overhead and reduced performance during write operations.
• Regularly analyze indexes. Monitor index effectiveness using MySQL tools like ANALYZE TABLE and adjust or remove unnecessary indexes to maintain optimal performance.
• Consider other index types. Explore unique indexes for enforcing uniqueness and full-text indexes for text search applications.

Managing Indexes

To modify or remove an index, use the following syntax:

Modifying an Index

Modifying an index generally requires dropping the existing index and creating a new one with the desired changes.

Dropping an Index

DROP INDEX index_name
ON table_name;

This command removes the index named index_name from table_name, which can be useful for managing performance or when the index is no longer needed.