How to Check and Manage Indexes in MySQL: SHOW INDEX, INFORMATION_SCHEMA, and EXPLAIN Guide

1. What Is a MySQL Index? The Key to Improving Database Performance

By effectively using indexes in a MySQL database, you can significantly improve query performance. An index is a data structure created on specific columns in a database table to enhance search and filtering speed. For example, when extracting specific information from a large dataset, using an index allows MySQL to skip scanning all rows and instead search only the indexed column.

Roles and Types of Indexes

MySQL provides the following types of indexes:

• PRIMARY (Primary Key): A unique key allowed only once per table, used as the main identifier of the table.
• UNIQUE Index: An index that enforces uniqueness and prevents duplicate values from being inserted into the specified column.
• Regular Index: An index without a uniqueness constraint, used to improve search performance on specific columns.

As shown above, indexes improve the efficiency of searches and data operations on tables and are essential for large datasets. However, having too many indexes can slow down INSERT and UPDATE operations, so proper index management is important.

2. Basic Methods to Check Indexes in MySQL

In MySQL, you can use the SHOW INDEX command to check existing indexes. This simple SQL command displays index information for a specified table. The specific procedure is explained below.

Basic Syntax and Output of SHOW INDEX

SHOW INDEX FROM table_name;

Explanation of Output Columns

When you execute this command, the following information is displayed:

• Table: The name of the table where the index exists
• Non_unique: Indicates whether the index allows duplicates (1) or is unique (0)
• Key_name: The name of the index
• Column_name: The name of the column to which the index is applied
• Cardinality: An estimate of the number of unique values in the index. This is used as an indicator of search efficiency.

Using this information, you can visually understand the index status within a table and how indexes are applied to each column. You can also narrow down the results using a WHERE clause if necessary.

3. Checking Indexes Using the INFORMATION_SCHEMA.STATISTICS Table

In addition to the SHOW INDEX statement, MySQL allows you to check indexes by querying the INFORMATION_SCHEMA.STATISTICS table. This method is useful for listing indexes across an entire database and provides more detailed information.

Basic Query for INFORMATION_SCHEMA.STATISTICS

SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME, INDEX_NAME 
FROM INFORMATION_SCHEMA.STATISTICS 
WHERE TABLE_SCHEMA = 'database_name';

Details of the Query Result

• TABLE_SCHEMA: The name of the database to which the index belongs
• TABLE_NAME: The name of the table where the index exists
• COLUMN_NAME: The name of the column to which the index is applied
• INDEX_NAME: The name of the index

This method allows you to view index information across multiple tables or a specific database in a single list. It is especially useful when managing indexes across an entire database.

4. How to Add and Remove Indexes and Their Impact

How to Add an Index

You can add indexes later as needed. Use the following command to create an index on a specified column:

CREATE INDEX index_name ON table_name(column_name);

For example, if you want to add an index to the email column of the users table, execute the following:

CREATE INDEX idx_email ON users(email);

How to Remove an Index

You can remove unnecessary indexes to optimize INSERT and UPDATE performance. Use the DROP INDEX command to delete an index:

DROP INDEX index_name ON table_name;

An example of an unnecessary index is one created on a column that is not used in search conditions (WHERE clauses). Removing such indexes can improve data insertion and update speed.

5. Checking Index Performance Using the EXPLAIN Statement

The MySQL EXPLAIN statement is useful for checking the query execution plan and identifying which indexes are being used. This helps evaluate index effectiveness and optimize performance when necessary.

Basic Usage of the EXPLAIN Statement

EXPLAIN SELECT * FROM table_name WHERE column_name = 'condition';

Using this command, you can determine whether an index is being used or if a full table scan is performed. The result includes the following columns:

• type: The query type (ALL indicates a full table scan; INDEX indicates an index is used)
• possible_keys: A list of indexes that could be used for the query
• key: The name of the index actually used
• rows: The estimated number of rows scanned

Based on this information, you can analyze the effectiveness of indexes and determine whether search performance improvements are necessary.

6. Conclusion

Proper index management is essential for optimizing MySQL database performance. Especially for tables handling large volumes of data, setting indexes on columns used in WHERE clauses and JOIN operations can dramatically improve search efficiency. However, having too many indexes can slow down insert and update operations, so maintaining an appropriate balance is important.

By understanding how to add, check, remove, and evaluate indexes, you can optimize your database more easily and improve overall system efficiency.