To understand a Composite Index, we must first recall that an Index is a data structure that enhances the speed of operations in a table. Typically, when creating an index, you specify a specific column of a table to speed up queries involving that column. A composite index, also known as a multiple-column index, includes not just one, but up to 16 columns.

Let’s illustrate this with an example:

Suppose we have a table called “kids_toys” and we want to include three columns in our index. The SQL statement would look like this:

CREATE TABLE kids_toys (toy_id INT PRIMARY KEY, toy_name VARCHAR(255), company_brand VARCHAR(255), age_restriction INT, INDEX idx_kids_toys (toy_name, company_brand, age_restriction) );

If the table is already created and you need to add the index, you would use:

CREATE INDEX idx_kids_toys ON kids_toys(toy_name, company_brand, age_restriction);

Now, consider a scenario where the index is already created on the table but contains only one column:

CREATE TABLE kids_toys ( toy_id INT PRIMARY KEY, toy_name VARCHAR(255), company_brand VARCHAR(255), age_restriction INT, INDEX idx_kids_toys (company_brand) );

In this case, you would use an ALTER statement to transform your INDEX into a Composite Index:

ALTER INDEX idx_kids_toys ON kids_toys (toy_name, company_brand, age_restriction);

With the composite index, you can efficiently search the following column combinations in the specified order:

• (toy_name)
• (toy_name, company_brand)
• (toy_name, company_brand, age_restriction)

Here is an example of a query using the index:

SELECT toy_id, toy_name FROM kids_toys WHERE toy_name = 'Legos';

And a query that won’t use the index:

SELECT toy_id, company_brand FROM kids_toys WHERE company_brand = 'Legos';

The second query doesn’t utilize the index because “company_brand” isn’t the first column in the index. Queries must follow the order defined in the index.

In summary, a composite index can significantly improve query performance, especially in large tables with complex queries. However, it’s important to use them judiciously, as they can also increase the storage requirement and affect insertion speed.